The prognostic significance of chromosome 18q allelic loss was evaluated in a series of 118 patients with curatively resected TNM stage II or stage III colon cancer. Chromosome 18q status was determined on frozen tumour samples, using microsatellite markers and the polymerase chain reaction (PCR). Mean follow-up in surviving patients was 75.9 months. Chromosome 18q allelic loss was significantly related to tumour site, extramural venous invasion, flow cytometric nuclear DNA content and p53 protein expression. Patients whose tumour had no evidence of chromosome 18q allelic loss showed a better disease-free and overall survival than patients whose tumour demonstrated 18q allelic loss. When patients were stratified by tumour stage, a significant survival advantage for patients whose tumour had no allelic loss on chromosome 18q was observed in stage II as well as in stage III disease. In particular, patients with stage II disease whose tumour had no chromosome 18q allelic loss demonstrated an excellent clinical outcome, with a 5-year disease-free survival rate of 96%. In contrast, the 5-year disease-free survival rate of patients with stage II disease and chromosome 18q allelic loss was only 54%. In multivariate analysis, status of chromosome 18q was the only significant independent prognostic factor for both disease-free and overall survival. These results indicate that assessment of chromosome 18q status provides relevant prognostic information in colon cancer and might be employed in the selection of patients for adjuvant therapy.