Female smokers who indulge the habit before the menopause, and especially before giving birth, might be increasing their risk of developing breast cancer.
A study by Harvard Medical School, Boston, found that smoking among younger age groups led to a "modest" rise in breast cancer. Their findings have been published in the journal Archives of Internal Medicine.
The scientists said: "Smoking before menopause was positively associated with breast cancer risk."
They also revealed that smoking post-menopause actually reduced the risk of breast cancer, possibly by restricting the levels of the hormone oestrogen.
Smoking might be a well known factor for other cancers, but its relationship with breast cancer has been hotly contested.
Previous studies have given mixed results, with some suggesting that smoking might protect against the disease.
The new research is based on data from the Nurses' Health Study, a major US investigation of the factors that affect women's health.
No significant association was seen between passive smoking, or light and moderate smoking, and breast cancer. But women who smoked heavily and took up the habit early in life were at increased risk.
For those who started using tobacco before giving birth for the first time, smoking the equivalent of 20 cigarettes a day for 20 years raised the risk of breast cancer by 18%.