The clinical and radiological findings in a pair of sibs with microcephalic osteodysplastic primordial dwarfism (MOPD) are described, a boy who survived for 5 1/2 years and his more severely affected younger sister, who died at the age of 6 months. Neuropathological studies in this girl showed marked micrencephaly with severely hypoplastic, poorly gyrated frontal lobes and absent corpus callosum. Our observation supports the hypothesis that types I and III MOPD probably constitute a spectrum of one and the same entity and published data together with this report are consistent with autosomal recessive inheritance. The pathogenesis of this condition is as yet unknown, but its characteristics indicate a basic defect affecting cell proliferation and tissue differentiation.