A population survey of retinitis pigmentosa and allied disorders in the Danish population disclosed 1301 cases prevalent in Denmark per January 1, 1988. Crude prevalence rates and World Standardized prevalences of systemic and non-systemic RP are presented. Sixty-four per cent of all RP-cases were non-systemic. A preponderance of males was encountered in both systemic and non-systemic RP. Usher disease was diagnosed in 12%, Bardet-Biedl syndrome constituted 5%, and the frequency of Spielmeyer-Vogt disease was 1% of all prevalent RP-cases. Other defined syndromes occurred in 1% of the cases. The Danish figures with respect to Usher disease are compared with standardized prevalence rates from Sweden and Finland. Nineteen per cent of patients affected by non-systemic RP had an onset later than 30 years of age. In patients affected by systemic RP the eye disease tended to an earlier debut with only a few per cent beginning after the age of 30 years.