The clinical expression of hereditary motor and sensory neuropathy type I (HMSN I) is age-dependent. Autosomal dominant HMSN I is heterogeneous at a molecular level with genes localized on chromosomes 1, 17, and possibly other chromosomes. In order to define accurately the penetrance of a single HMSN I gene mutation, we performed nerve conduction studies in HMSN I families whose genetic defect was linked to chromosome 17 (HMSN Ia). All HMSN Ia subjects tested had slow nerve conduction velocities with a mean median velocity 20 +/- 6 m/sec, which did not change with age. The range of conduction velocities from affected individuals did not overlap those from their clinically normal relatives, indicating complete penetrance of the gene from early childhood. The results indicate that motor nerve conduction studies in children can add additional information for linkage studies and genetic counseling.