We studied 23 families with "pure" autosomal dominant spastic paraplegia. Examination of 142 at-risk individuals allowed identification of 70 patients, including 12 who were clinically affected but unaware of symptoms. The frequency of lower limb muscle weakness, decreased vibration sense, hyperreflexia in the upper limbs, and sphincter disturbances increased with the disease duration. The distribution of age at onset was unimodal, with a mean onset of 29 years (range, 1 to 68). The clinical manifestations of "early-onset" (< 29 years) and "late-onset" (> 29 years) patients were not significantly different. Age at onset varied as much within families as among families; anticipation and imprinting did not occur. No clinical criteria allowed differentiation among the families studied. Only linkage studies can provide accurate classification of this disease.