The Coffin-Lowry syndrome is an established syndrome of mental retardation, a characteristic facies and skeletal anomalies. This paper describes 12 cases from eight families and compares their findings with those of previously reported patients. The differential diagnosis is considered. Physical findings and pedigree data strongly support X-linked semi-dominant inheritance. The gene appears widely distributed and, as expected, a significant proportion of cases represent new mutations. We cannot confirm the metacarpal-phalangeal profile or fingertip dermatoglyphics as useful diagnostic aids. Skin biopsy studies from four of our patients gave no evidence for a primary disorder of lysosomes or a degenerative disease. Caution is urged before assuming that such patients will all show intellectual deterioration.