We analyzed clinical data of 569 patients in two combined series with childhood and juvenile proximal SMA. This cohort included only patients who had achieved the ability to sit unaided (type II and III SMA). The survival rate among 240 type II patients (who sat but never walked) was 98.5% at 5 years and 68.5% at 25 years. SMA III (n = 329) (those who walked and had symptoms before age 30 years) was subdivided into those with an onset before and after age 3 years (type IIIa, n = 195; SMA IIIb, n = 134). In patients with SMA III, life expectancy is not significantly less than a normal population. The probabilities of being able to walk at 10 years after onset was 70.3%, and at 40 years, 22.0% in SMA IIa. For SMA IIIb, 96.7% were walking 10 years after onset and 58.7% at 40 years. The subdivision of type III SMA was justified by the probability of being ambulatory depending on age at onset; the prognosis differed for those with onset before or after age 3 years. The data provide a reliable basis of the natural history of proximal SMA and support a classification system that is based primarily on age at onset and the achievement of motor milestones.