Using in situ hybridization with a chromosome-specific DNA probe, we investigated the numerical aberrations of chromosome 17 in differentiated gastric cancers. In 75 cancers that invaded the submucosal or deeper layer, the frequency of an increased hybridized signal (hyperdiploid nuclei) on chromosome 17 was significantly associated with lymph node metastasis. Multivariate analysis showed that the classification of numerical aberration (disomy and polysomy) based on the percentage of hyperdiploid nuclei was an independent prognostic factor for the postoperative survival of 58 patients with invasive gastric cancer treated with radical gastrectomy. Thus, numerical aberrations of chromosome 17 may be a useful prognostic indicator of differentiated gastric cancer.