Rickets, a bone-weakening disease of children caused by a lack of vitamin D, disappeared from the U.S. about 50 years ago, following the introduction of vitamin D-fortified foods. So most American pediatricians have never seen a case of rickets - until recently.

Dr. Suruchi Bhatia saw her first case two years ago, when an African immigrant family brought their son to Children's Hospital in Oakland, California. "This child looked small for his age," Dr. Bhatia recalls. "He had slightly bowed legs. He was clingy and not wanting to get down on the floor and walk."

Although the endocrinologist hadn't seen it before, she immediately recognized the problem. "I said, 'Oh, O-K, this is vitamin D-deficiency rickets.'"

Much to the endocrinologist's surprise, over the next year she started to see dozens of children who were not immigrants from Africa, kids that were native Oaklanders, with the same bent legs and stunted growth. Bhatia wondered why these youngsters weren't getting enough vitamin D, or if there was something in the environment causing so much rickets. Vitamin D is also produced by skin exposed to sunlight, which, she points out, is often obscured in the San Francisco Bay area by fog. "I speculated that it was cloudier up here."

But Bhatia also suspected that the vitamin D deficiency was a side-effect of a tremendously successful public health campaign for breast-feeding. Baby formula is fortified with vitamin D. Breast milk has virtually none. And the rate of exclusive breast-feeding has increased greatly in the United States over the past few years.

A few blocks from Bhatia's office, at the federal feeding program for nutritionally at-risk women, infants and children known as WIC, a young mother explains why she chose to breast-feed. "I heard it was really healthier for the baby," she says with a smile, "and I've been coming to all my WIC appointments, and they tell me it's really great. So my baby will be healthy."

Denise Barkasy, a certified lactation counselor for WIC, agrees. "In the last few years, we have been successful in encouraging these moms to breast-feed," she says, adding "Breast milk is the perfect food for you and your baby."

Except that it lacks vitamin D. Bhatia concluded that the recurrence of rickets was caused by the surge in breast-feeding.

But every ethnic group was breast-feeding more, while rickets was primarily affecting just one group. "The bulk of them were African American," Bhatia notes, "about 75 percent, 80 percent African American children." Kids with darkly pigmented skin produce only about 5 percent of the vitamin D of children with lighter skin. The result, if they're breast-fed with no vitamin D supplements? Rickets.

But there's nothing new about dark-colored skin or breast-feeding. So Bhatia searched for another explanation for the return of rickets, and found it in the American life-style. "Often it's the case that both parents are working and the children are indoors at a daycare until they're picked up and brought home in the evening when it's dark already," she observes. American kids are also indoors more because of TV, video and computer games. And in Oakland, says Bhatia, violence keeps them out of the sun. "When you watch the evening news and you hear of two toddlers shot as they're playing in the front yard, you can understand why most parents wouldn't be eager to have their children playing outside in the front."

The return of rickets in the United States is not limited to California. Articles about how to diagnose and treat rickets began appearing in medical journals as physicians in other states recognized the problem. Dr. Robert Schwartz, professor of pediatrics at Wake Forest University in North Carolina, found 30 cases in his state. He talked with colleagues nationally, "and we found it was increasing around the country. And the more we started talking with people, the more cases we found. Everyone was seeing this."

The federal Centers for Disease Control has been studying the increased reports of rickets around the country. Epidemiologist Kelly Scanlon estimates that nationally as many as one in 10,000 African American children under five have the disease. And, she says, she's concerned. "It is a serious condition and one that we know how to prevent. And it also disproportionately affects children with darker skin."

In 2003, the American Academy of Pediatrics acknowledged the rise in rickets by telling pediatricians to recommend daily vitamin D supplements for their patients. But in the most recent survey, only half of pediatricians were following the recommendation.

North Carolina's Dr. Schwartz says the federal government has been equally lax, refusing to recommend or provide Vitamin D to mothers in the WIC programs. So North Carolina decided to do it with state funds. "We began distributing a multivitamin supplement free to the mothers of all infants who were exclusively breast-feeding their infants, and who were coming to the WIC clinics," he explains. "It cost about $1.50 a month, and so that we see a marked decrease in cases in our state." He says pediatricians now see only an occasional case.

Schwartz says there's a simple cure for the rickets resurgence in North Carolina, California and the nation. If state and federal programs provided vitamin D to breast-fed babies, rickets might once again disappear.