The rumor that antiperspirant use causes breast cancer continues to circulate the Internet. Although unfounded, there have been no published epidemiologic studies to support or refute this claim. This population-based case- control study investigated a possible relationship between use of products applied for underarm perspiration and the risk for breast cancer in women aged 20-74 years. Case patients (n = 813) were diagnosed between November 1992 and March 1995; control subjects (n = 793) were identified by random digit dialing and were frequency-matched by 5-year age groups. Product use information was obtained during an in-person interview. Odds ratios (ORs) and 95% confidence intervals were estimated by the use of conditional logistic regression. P values were determined with the Wald chi(2) test. All statistical tests were two-sided. The risk for breast cancer did not increase with any of the following activities: 1) antiperspirant (OR = 0.9; P =.23) or deodorant (OR = 1.2; P =.19) use; 2) product use among subjects who shaved with a blade razor; or 3) application of products within 1 hour of shaving (for antiperspirant, OR = 0.9 and P =.40; for deodorant, OR = 1.2 and P =.16). These findings do not support the hypothesis that antiperspirant use increases the risk for breast cancer.