Two hundred and seven large bowel cancer patients (93% of all cases diagnosed in a defined community between 1965 and 1976) were matched at random with non-cancer subjects of same age, sex, and place of residence. Men with cancer, aged 75 years and younger than, had a more frequent history of work in a local factory handling synthetic fiber than controls (22 versus 10; P less than 0.025). In this factory 45% of cancers occurred before age 60, while this was true in only 24% of cancer cases outside the factory (P less than 0.05). There was a greater tendency for cancers in factory workers to occur in the colon than in the rectum. Heredity was not found to be a risk factor for large bowel cancer. The rate of prior appendectomy was higher in men with colon cancer (P less than 0.05) and the rate of prior cholecystectomy was lower in females with colonic cancer (P less than 0.05). A previous hemorroidectomy was also found more often in males with colonic cancer (P less than 0.05). Long-standing severe constipation was present more often in patients with cancer (P less than 0.01). There was some evidence for a compounding influence of different risk factors, as studied by relative risk ratio. This study confirms the existence of a high risk of large bowel cancer in a group of workers in a synthetic fiber factory and suggests other factors antecedent to large bowel cancer.