The relationship between first degree family history of colorectal cancer and the risk of benign or malignant tumours of the large bowel was investigated in a case-control study. Two groups of cases (283 patients with adenomatous polyps and 414 patients with adenocarcinoma of the large bowel) and 2 groups of controls (399 polyp-free subjects and 456 hospitalized patients) were interviewed. Since no difference in the frequency of family history between the 2 control groups was detected, these were lumped together. A 3-fold increase in risk of adenomatous polyps in relatives of patients with colon cancer was observed (OR = 3.18, 95% CI 2.06-4.89). The relative risk of colorectal cancer among relatives of patients with adenocarcinoma was 2.36 (95% CI 1.54-3.60). No significant difference in the frequency of first degree relatives with a history of cancer of the large bowel was detected between patients with colorectal cancer and those with adenomatous polyps. When only history of colorectal cancer among parents was considered, the results closely paralleled those of the previous analysis.