Cigarette smoking has been hypothesized to have a carcinogenic effect on breast tissue, particularly when exposure occurs before sexual maturity and breast tissue differentiation (1). Epidemiologic studies of cigarette smoking on breast cancer risk during this developmental period have produced inconsistent results (2-14). However, recent cohort studies (7-10), including two with mainly young women (7, 10), suggest an effect of smoking at an early age. In this study, we examine the effect of smoking on breast cancer risk in women <50 years of age.

Details of this population-based case-control study have been described elsewhere (15). Briefly, 1,794 U.S.-born, English-speaking, non-Hispanic white, Hispanic white, and African-American Los Angeles female residents with incident invasive breast cancer, ages 20-49 years, were recruited through the Los Angeles County Cancer Surveillance Program between February 1998 and May 2003. Four hundred forty-four...

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