Complete ascertainment of fibrodysplasia ossificans progressiva in the United Kingdom was attempted and 44 patients were identified. This indicates a point prevalence of 0.61 x 10(-6). The disease is determined as an autosomal dominant trait which has complete penetrance but variable expressivity. No evidence for genetic heterogeneity was found in this series. All patients represented fresh gene mutations and their biological fitness was zero. Geographical clustering of these new mutations was evident but conformed to the general population distribution. The direct estimate of the mutation rate was 1.8 (SE +/- 1.04) x 10(-6) mutations per gene per generation. A significant paternal age effect was evident for these new mutations in the United Kingdom.