A functional polymorphism at codon 33 (leucine-to-proline, Leu33Pro)/nucleotide 1565 (T-to-C, T1565C) of the integrin beta3 has been hypothesized to increase the risk of breast cancer and its metastasis. Three studies have been conducted up to date and the results were contradictory. We used a large population-based age-matched case-control study in German Caucasian women by the age of 50 years to assess breast cancer risk associated with this polymorphism, taking into consideration of possible interaction with other risk factors, and to examine if it affects clinical presentation. Overall, the odds ratios (OR) for breast cancer were not increased in women carrying either allele. However, we observed a differential effect of the Leu33Pro polymorphism by age group when patients were stratified by 45 years of age (p=0.055). Being a carrier of the 33proline allele was found to be associated with a 32% increased risk (95% Cl=1.0-1.8) for breast cancer compared to the wild-type leucine homozygotes among women age 45 or younger but not in older women. Furthermore, we observed significant dose effect of the 33proline allele (p=0.04), with 30% risk increase per allele (95% Cl=1.0-1.7). Significant evidence was also found for a positive association between 33proline carrier status and increasing axillary node involvement (p=0.048) but neither size nor grading of tumor in this study. Our data suggest that inheritance of the integrin beta3 Leu33Pro polymorphism may increase the breast cancer risk by age 45 in the German population.