Autosomal dominant spinal muscular atrophy (SMA) is generally classified into a juvenile and an adult onset form. Clinical data of 20 affected members out of 6 families with autosomal dominant proximal SMA are reported. Three families could largely be classified as the adult onset form (onset after 20 years of life). They showed a benign course, most of them remaining ambulatory 10-40 years after clinical onset. Intrafamilial variability of onset was small, the progression of weakness within one family appeared to be very similar. Three patients of the other 3 families suffered from the juvenile onset form (first symptoms before the age of 12 years) with walking difficulties throughout life, whereas other family members would have been classified as adult onset SMA. The latter had an onset between age 17 and 28 years, and were only moderately handicapped when last examined (aged 38-60 years). The great intrafamilial variability in at least some of the families with autosomal dominant SMA is not compatible with the distinction of two clinically defined genetic entities. This observation is important with respect to a reliable prediction in clinical practice and genetic counselling.