The incidence and sex ratio of transsexualism in Sweden were calculated during the period between 1 July 1972 and 30 June 1992 using a case register kept on file at the Swedish Bureau of Social Welfare. Only cases of primary/genuine transsexualism were considered. The study is unique in that it has been performed in exactly the same way as a previous study by one of the authors (JW) in the 1960s and early 1970s. The data can therefore be reliably compared. The results show that the incidence figures remain constant over time and that the incidence of primary/genuine transsexualism is equally common in men and women. A larger group consisting of all those individuals who had applied for sex reassignment revealed a preponderance of men. Analysis showed that this large group included not only primary/genuine transsexuals but also effeminate homosexuals, transvestites and diagnostically uncertain cases. The importance of describing the exact methods used when calculating epidemiological data is highlighted.