Reports suggesting an increasing incidence of male genitourinary anomalies such as hypospadias, possibly related to environmental factors such as environmental estrogen-like compounds, have recently received considerable publicity. These reports are based on birth defects registry data, and there may be variation in the completeness of the registries used. We analyzed temporal trends in the prevalence of hypospadias in Finland to assess the previously reported low overall prevalence and to detect any possible increasing tendencies during the past decade. We identified all patients who were surgically treated for hypospadias before the age of 9 years among boys born 1970-1986 in the national hospital discharge registry. We calculated the cumulative prevalence by dividing the number of patients by the number of male births, and we used Poisson regression analysis. Out of 549,176 boys born in Finland in 1970-1986, 1,543 were treated for hypospadias by the age of 8 years (28.1 surgically treated patients per 10,000 male live births; 95% confidence interval, 26.7-29.5). The prevalence of hypospadias in Finland remained constant throughout the study period and appears to have been approximately three times higher than previously reported. Changes in completeness of registration may account for a substantial proportion of the reported increases in the prevalence of hypospadias in Finland and possibly also elsewhere.