Between 1976 and 1980, medical and social service sources were used to ascertain cases of retinitis pigmentosa in Maine (1980 population, I, 124,660). As of July 1, 1980, 241 clinically prevalent cases of retinitis pigmentosa were ascertained. Extensive pedigrees were collected for 185 of the subjects and medical records were obtained. One hundred fourteen cases were further evaluated by clinical examination including electroretinography. Adjusting for incorrect diagnosis (eight of 114, 7%) and underascertainment (23 of 185, 12.5%), we estimated that prevalence of retinitis pigmentosa in Maine is 236 cases, 21 per 100,000 population or 1:4,756. Excluding Usher and Bardet-Biedl syndromes, the prevalence is 1:5,193. Estimated birth incidence of persons who will become affected with non-syndrome retinitis pigmentosa is 1:3,544. Incidence of newly diagnosed cases per year is about six per 1,000,000 population. Among kindreds, 16 of 85 (19%) were autosomal dominant, 55 of 85 (65%) autosomal recessive or isolated cases, seven of 85 (8%) X-linked recessive, and seven of 85 (8%) not classified by mode of transmission.