A mother of normal intelligence and her moderately mentally retarded son, both with the typical facial features of the Brachmann-de Lange syndrome, are reported. We discuss the variable expression of the Brachmann-de Lange syndrome by comparing the autosomal dominant cases with the sporadic or presumed autosomal recessive cases. The autosomal dominant cases show milder symptoms in general. In our opinion, a de novo autosomal dominant mutation causes the severe form of the syndrome, recurrence within sibships being explained by germline mosaicism. In all convincingly autosomal dominant cases we found that the mother is the transmitting parent, suggesting genomic imprinting.