The Rubinstein-Taybi syndrome is a condition characterized by mental retardation, typical facial changes and broad thumbs and big toes. The cause is unknown; almost all cases are sporadic. We describe a mother and son with Rubinstein-Taybi syndrome. Literature search documented at least 413 cases with 558 sibs. An affected offspring has occurred at least twice, possibly six times. In 45 yet undescribed Dutch cases no recurrence was found in 94 sibs. A submicroscopic chromosome deletion seems the most probable explanation, but other alternatives may be possible.