To investigate the genetic factor in the development of colorectal cancer, a cohort study was undertaken of parents of patients with this disease. All 1,524 patients, who were diagnosed with colorectal cancer in Denmark in 1982-84 and were below the age of 60 years, were selected from the National Cancer Register. The parents of these patients were identified from the public population registers. The index persons had a total of 1,478 eligible mothers, of whom 96% were successfully traced, and a total of 1,414 eligible fathers, of whom 96% were traced. These parents were sought in the Cancer Register for cancer cases diagnosed in the period January 1, 1943 to December 31, 1986. The incidence rates for the Danish population were used to estimate the expected numbers of colorectal cancer cases among the parents. Both the mothers and the fathers exhibited an increased risk of colorectal cancer. The standardized incidence ratios were 1.62 (95% CI 1.31-2.01) and 1.87 (95% CI 1.54-2.27), respectively. In a previous study we found that spouses of patients with colorectal cancer in Denmark did not have an increased risk of this disease. The increased risk found in the present study among the parents of patients therefore indicates that a possible genetic factor is present in the aetiology of colorectal cancer, and that it is of importance in the general population.