BOSTON - Lavender and tea tree oils found in some shampoos, soaps and lotions can temporarily leave boys with enlarged breasts in rare cases, apparently by disrupting their hormonal balance, a preliminary study suggests.

While advising parents to consider the possible risk, several hormone experts emphasized that the problem appears to happen infrequently and clears up when the oils are no longer used. None of those interviewed called for a ban on sales.

The study reported on the condition, gynecomastia, in three boys ages 4, 7 and 10. They all went back to normal when they stopped using skin lotions, hair gel, shampoo or soap with the natural oils.

It's unclear how often this problem might crop up in other young children.

These plant oils, sometimes called "essential oils," are added to many health-care products, usually for their scent. The oils are sometimes found in other household products or sold in purer forms. Tea tree oil is sometimes used in shampoos for head lice.

The suspected effect in this study is blamed on some chemical within the oils that the body processes like estrogen, the female hormone that promotes breast growth.

The findings were being reported Thursday in the New England Journal of Medicine. The federally funded study came out of the University of Colorado and the environmental health branch of the National Institutes of Health. The findings were first released last year at a science meeting.

The three boys were brought to their doctors with overdeveloped breasts that looked like those of girls in early puberty. They were sore in one case. For each boy, doctors could tie the problem only to their use over several months of the natural-oil products.

The researchers suspected that the oils might be upsetting the boys' hormonal balance. So they did a series of laboratory tests to check how these oils work within human cells. The oils appeared to mimic estrogen and block the male hormone androgen.

On product labels, the oils sometimes are listed by their scientific names: Lavandula angustifolia (lavender oil) and Melaleuca alternifolia (tea tree oil). Such products do not require government approval to be sold unless they make specific health claims.

Marijuana and soy products also have been linked to gynecomastia.

Dr. Clifford Bloch, a hormone specialist in Greenwood Village, Colo., who treated the three boys, recommended that parents "be cautious" with such products, especially for prolonged use. "I would not give these products to my children," he said in an interview.

Bloch said he also suspects the oil played a role in a handful of young girls he saw for a similar condition, including a 17-month-old whose parents were washing her bottles with a lavender-scented soap.

Others sounded less worried. "It takes very little estrogen to cause gynecomastia in a young child," said Dr. Richard Auchus, a University of Texas Southwestern Medical Center hormone expert who knew of the study findings. "If they're getting it for a brief period of time, that really shouldn't cause long-term problems."

Also, the research did not pinpoint any specific estrogen-like compounds in the oils or look for them in a range of products. Chemist Steven Dentali, at the industry group American Herbal Products Association, said that warning people to avoid such oils "is premature without the additional basic research needed to bolster the case that the issue here is both real and significant."

Gynecomastia is very common in boys during the hormonal changes of puberty. But it also occurs as a rare condition in younger boys, men, and girls before puberty.

Bloch, the study doctor, said it's unknown if such oils could hurt women with estrogen-fed breast tumors.