The natural history of SMA and the identification of predictive criteria of functional development are still a matter of discussion. This prospective study involved 20 children with SMA, aged between 3.10 and 15.7 years. The patients were followed from 1979 to 1992 in order to try to develop a greater understanding of the natural history of SMA at a very early age. A standardized protocol was used at regular intervals to assess parameters such as joint contractures, scoliosis and the milestones of gross motor functions. Our findings agree with the data in the literature concerning the early and generalized onset of joint contractures and scoliosis. A significant correlation was found between the level of acquired gross motor functions and walking with support. The acquisition of rolling by 5 years of age was the milestone that best correlated with the acquisition of walking with crutches and braces; furthermore the inability to roll seemed to correlate with the severity of the disease. These findings should be useful in planning a more rational rehabilitation program.