Exposure to high levels of sex hormones before birth, which can induce both left-handedness and changes in breast tissue, may explain the apparent link.

The research, conducted in the Netherlands, involved more than 12,000 middle-aged women born between 1932 and 1941 who were healthy at the onset of the study.

The researchers took their body measurements and recorded information concerning such risk factors as social and economic status, smoking habits, family history of breast cancer and reproductive history.

They followed a random sample of 1,500 of the women via the Dutch regional cancer registry until January 2000.

39% Higher Risk Overall

Overall, the investigators observed a 39 percent higher risk for developing breast cancer in left-handed women. Adjusting for risk factors hardly affected the overall association, they report.

They found that left-handed women were more than twice as likely to develop premenopausal breast cancer as non-left handed women. There appeared to be no excess risk for postmenopausal breast cancer, however.

The reseaarchers observed the link in left-handed women with a body mass index (BMI) of 25 or less -- considered normal weight -- but not in those whose BMI exceeded that number.

Further, the increased risk level associated with left-handedness was seen only in women who had given birth.

Exposure to Sex Hormones

Exposure to high levels of sex hormones before birth, which can induce both left-handedness and changes in breast tissue, may explain the connection, the authors speculate.

"Although the underlying mechanisms remain elusive," they note, "our results support the hypothesis that left-handedness is related to increased risk of breast cancer."