The I1307K mutation of the adenopolyposis coli gene (APC), located on chromosome 5q21-q22, is associated with an increased risk of cancer in Ashkenazi Jews. In the present study, we analyzed age and body mass of Ashkenazi Jewish prostate cancer patients, with and without the APC I1307K mutation. Participants in our study were found through urology and radiation oncology clinics, and all eligible patients were asked to take part. A familial history was obtained by interview or self-report questionnaire. Histological confirmation of diagnosis was obtained for all subjects. The I1307K allele of the APC gene was detected by amplification of lymphocyte DNA from peripheral blood according to standard polymerase chain reaction (PCR) and dot blot procedures. We studied 135 Ashkenazi Jewish men with prostate cancer. The youngest was 49, the oldest 80, average age 68 +/- 6.88 (mean +/- SD). The older patients carrying the wild type APC allele tended to have a lower body mass than the younger ones (r = -.27, P =.002). Of 71 patients under 70 years old, 65 carried the wild type APC allele, and had a body mass index of 28. 7 +/- 4.23 kg/m(2). The six men under age 70 carrying the I1307K APC allele had a body mass index of 26.87 +/- 1.44 kg/m(2). The difference in body mass index of the two groups is significant (P =. 032, t test for unequal variance). Increased body mass is a prostate cancer risk factor, and hereditary prostate cancer is associated with younger patients. Therefore, our finding, that patients under age 70 carrying the I1307K allele are significantly thinner than those carrying the wild type allele, suggests that the APC I1307K allele is also a prostate cancer risk factor. Our results are in accord with other studies indicating that APC mutations increase the risk of prostate cancer.