Epidemiologic data on 45 patients with Rubinstein-Taybi syndrome from the Netherlands and 50 patients from the USA are compared with data from 407 patients reported in the literature. The 502 probands had a total of 708 sibs, including one probable recurrence. In 12 of 13 proven or possible monozygotic twins both children were affected. Two patients have reproduced with one affected and 2 normal offspring. The empiric recurrence risk figure for sibs is 0.1%. The recurrence risk for offspring of affected individuals could be as high as 50%. The cause of the syndrome remains unknown. There were no clues for autosomal recessive or X-linked inheritance, nor for a teratogenic cause. No consistent chromosome anomaly was found. An autosomal dominant mutation, either as submicroscopic chromosome deletion or duplication, or a point mutation seems the most likely explanation.