In our recent cancer registry-based study, the incidence of gastric carcinoma was increased up to 5-fold in male relatives of early-onset prostate cancer (PCA) patients. This association may reflect the influence of genetic factors predisposing individuals to both tumor types. Germ-line mutations of the CDH1 gene at 16q have recently been associated with familial gastric cancer. Furthermore, two genome-wide linkage studies of PCA recently reported positivity at 16q. We therefore identified families and individual patients with both gastric and PCA and investigated whether the CDH1 gene mutations were involved in cancer predisposition in these cases. Fifteen of the 180 Finnish hereditary PCA families (8.3%) had one or more gastric cancer cases. No truncating or splice site CDH1 mutations were identified by PCR single-strand conformational polymorphism in these families or in eight individual patients who had both prostate and gastric cancer. However, a novel S270A missense mutation in exon 6 of the CDH1 gene was seen in a single family with four prostate and two gastric cancers. A large-scale population-based survey indicated a higher prevalence of S270A among both familial PCA cases (3.3%; n = 120; P = 0.01) and unselected PCA patients (1.5%; n = 472; P = 0.12) as compared with blood donors serving as population controls (0.5%; n = 923). We conclude that individual rare mutations and polymorphisms in the CDH1 gene, such as S270A, may contribute to the onset of PCA and warrant further investigations in other populations. However, the CDH1 gene does not appear to explain the link between prostate and gastric cancer.