Adenomatous polyposis syndrome (APS) is often associated with mental retardation, but whether there is a causal relationship between the two is unknown. This study was initiated to determine colorectal disease rates among mentally retarded subjects and whether they are at risk of developing colorectal cancer. Colonoscopic mass screenings were conducted to detect the presence of colonic neoplasms in 134 patients housed at an institution for the mentally retarded. The ages of these subjects ranged from 32 to 69 years (mean: 44 years) and their IQ scores ranged from 35 to 63. Screening presented no difficulties and the colonic regions up to the caecum were inspected in all patients. Polyps were found in 24 patients (17.9%), an abnormal fixation in two (1.5%), melanosis coli in two (1.5%) and haemorrhoids in two (1.5%). Thirteen subjects had a single polyp (54%), five had two polyps (21%), three had three polyps (12.5%) and three had four polyps (12.5%). Twenty one polyps were less than 5 mm in diameter, 14 were 5-9 mm and nine were 10 mm or larger. No cases of APS were detected during this mass screening programme. The mentally retarded patients in this study were not found to be at a high risk of developing either APS or colon cancer.