Epidemiologic studies of ulcerative colitis in western countries have shown that former smokers are at greater risk than those who never smoked, but current smoking and alcohol drinking habits appear to be inversely associated with the development of this disease. The present case-control study examines whether these relations are found among Japanese people, after adjustment for the separate effects of smoking and alcohol consumption. The subjects were 384 patients with ulcerative colitis who first received financial aid for treatment of this disease within the 2-year period from April 1988 to March 1990. From the same public health centers, age- and sex-matched paired controls were selected randomly from rosters of persons attending health checkup programs. Information on smoking behavior and alcohol use was obtained by self-administered questionnaires. Compared with nonsmokers, former smokers were at increased risk of suffering from this disease (odds ratio (OR) = 1.67, 95% confidence interval (CI) 0.97-2.88); current smokers were at decreased risk (OR = 0.30, 95% CI 0.18-0.50); and usual consumption of alcohol reduced the risk compared with less frequent use (OR = 0.57, 95% CI 0.37-0.86). Significant dose-response relations were observed between these two factors and the disease. In conclusion, the relations between both smoking and alcohol consumption and ulcerative colitis among Japanese people exist independently and are consistent with those reported in several previous studies outside Japan.