A new study has found a significant link between women’s exposure to DDT as young girls and the development of breast cancer later in life.

The results are something of a surprise, researchers said, because several previous studies have found no link between cancer and the insecticide, which was widely used during the 1950s and ’60s but was banned in the United States in 1972.

The new work differs from all other studies, however, by focusing on the age at which women were exposed. Echoing the situation with some other breast cancer risks, such as radiation, it finds that DDT increases the risk of breast cancer in adulthood only if the exposure occurred at a young age, before the breasts were fully developed.

All told, girls who had the highest levels of the chemical in their blood during that crucial developmental period were five times more likely to get breast cancer years later than were girls who had the lowest levels. That fivefold increase is a bigger boost in risk than is now attributed to hormone replacement therapy or having a close relative with breast cancer.

Although there is nothing that women today can do about their DDT exposures decades ago, the results could influence an ongoing controversy about the extent to which the chemical should still be used around the world.

That question has haunted the World Health Organization (WHO) because, despite its environmental and potential human health risks, DDT remains one of the most potent weapons against the mosquitoes that transmit malaria, a global scourge that kills about a million people every year, most of them children.

“This is a very important public health issue that calls for a balanced approach,” said the leader of the new study, Barbara Cohn, director of Child Health and Development Studies, a nonprofit research initiative in Oakland, Calif.