We compared 46 patients having onset of Parkinson's disease before age 45 years with 52 having onset after age 70. Young-onset cases more often presented with muscular stiffness (43%) and old-onset with difficulty walking (33%). One-third of young-onset cases had off-period dystonia, mostly affecting the legs, but no dystonia was recorded in old-onset cases. Presentation with rest tremor occurred in 41% of young-onset and 63% of old-onset. There were no differences in the number of affected relatives, endocrine disease, personality characteristics, dementia, or dyskinesia. A pathological study of 12 young-onset and 22 old-onset cases showed 24% greater nigral cell loss in the young, but no differences in the basic Lewy body pathology. Median disease duration in young cases was 5 years longer in the clinical study and 12 years longer in the pathological study. These studies show that the Parkinson's disease process is similar in young- and old-onset cases.