One hundred and thirty two individuals at risk for hereditary motor and sensory neuropathy (HMSN) type I from 11 unrelated families were evaluated by physical examination. Motor conduction velocity (MCV) studies of median and/or peroneal nerves were performed on 99 of them. Seventy-three subjects were found to be affected. In all age categories including the first decade of life, the ratio of affected individuals at risk did not significantly differ from the expected 1:1 ratio; that is, penetrance of the gene was complete. The majority of affected members in the first decade had no clinical features considered diagnostic of peroneal muscular atrophy syndrome, and full clinical expression developed in the second decade. Marked slowing of MCV was already present in the early years of life, even as young as 6 months. Moreover serial MCV studies carried out throughout the first year of life in an affected girl showed no physiological increase in conduction velocity. For purposes of genetic counseling, our experience suggests that, starting from 6 months of age, a clinically and electrophysiologically normal subject has a zero risk of having inherited the HMSN type I gene. However given the limited numbers in this series, infants at risk with normal clinical evaluation and MCVs should be followed up yearly up to 5 years of age.