To pursue a borderline increased risk of breast cancer for carriers of two integrin beta(3) (ITGB3) 33Pro alleles found in a recent prospective study, we conducted a case-control study of 1088 women with breast cancer and 4815 female controls. Leu33Pro heterozygotes, homozygotes and heterozygotes+homozygotes vs noncarriers had odds ratios for breast cancer of 1.0 (95% confidence interval: 0.8-1.1), 0.8 (0.5-1.2) and 1.0 (0.8-1.1), respectively. After stratification for conventional risk factors, odds ratio for breast cancer in heterozygotes, homozygotes and heterozygotes+homozygotes vs noncarriers were not increased above 1.0 in any of the 14 strata examined. This was also true after stratification for tumour histological subtype and cancer stage at the time of diagnosis.