The cause of the Rubinstein-Taybi syndrome (RTS), a multiple congenital anomalies/mental retardation (MCA/MR) syndrome first described in 1963, remains obscure. Recently, a deletion of chromosomal material at 16p13.3 has been found in some patients with the disorder, but no such deletion can be identified in the majority of affected individuals. Although the disorder has been well documented to be concordant in at least 7 monozygotic twin pairs and in one non-twin sib pair, only one clear-cut case of parent-to-child transmission has been reported previously. We present here a mother and daughter, both of whom appear to be affected with RTS, strongly suggesting either autosomal or X-linked dominant transmission. The paucity of previous cases of parent-to-child transmission may be related to either decreased fertility or decreased fitness in affected individuals.