Microcephaly, absent psychomotor development and dystonic limb movements were the main clinical features of a 3-year-old girl affected by hypoplasia of the pontocerebellar structures. As in the few previously reported cases there are discrepancies between the severity of lesions in the supratentorial and infratentorial compartments. Pathological features such as size reduction of the ventral pons, inferior olive atrophy, dentate nucleus fragmentation, and thinning of the cerebellar cortex suggest an impaired maturation of the involved structures due to a prenatal condition (dated at about 20-28 weeks of gestation). Somatotopic analysis failed to provide conclusive evidence on the primary target of the disease. The affected structures originate from the dorsal rhombencephalic region at about the same gestational age, and their maturation is probably under the control of sets of genes which regulate pattern formation. Early abnormal functioning of such genes might lead to the selected morphogenetical alterations observed in ponto-cerebellar hypoplasia. The normal morphogenetic pattern of the supratentorial structures and the mild lesions observed suggest that their late involvement can be related to a different pathogenetic process.