Information concerning bowel habits was gathered from a representative sample of 14,407 United States adults in the first National Health and Nutrition Examination Survey in 1971-1975 and approximately 10 years later among the same individuals. The prevalence of self-reported constipation, diarrhea, infrequent defecation (three or fewer bowel movements per week), and frequent defecation (two or more bowel movements per day) increased with aging. Women were more likely than men (P less than 0.05) to report constipation (20.8% compared to 8.0%) and infrequent defecation (9.1% compared to 3.2%). Blacks were more likely than whites to report infrequent defecation (P less than 0.05). Older respondents reporting constipation were more likely to use laxatives or stool softeners than younger respondents reporting constipation, but they were also less likely to have infrequent defecation. To evaluate factors predictive of impaired bowel function, case definitions were created using information concerning complaint of constipation, laxative use, frequency of defecation, and stool consistency. Female gender, black race, fewer years of education, low physical activity, and symptoms of depression were independent risk factors for impaired bowel function. This study provides national estimates of bowel complaints and their natural history and examines possible risk factors for constipation.