The aim of our longitudinal multicentric study was to establish the changes on the 6min walk test (6MWT) in ambulant SMA type III children and adults over a 12month period. Thirty-eight ambulant type III patients performed the 6MWT at baseline and 12months after baseline. The distance covered in 6min ranged between 75 and 510m (mean 294.91, SD 127) at baseline and between 50 and 611m (mean 293.41m, SD 141) at 12months. The mean change in distance between baseline and 12months was -1.46 (SD 50.1; range: -183 to 131.8m). The changes were not correlated with age or baseline values (p>.05) even though younger patients reaching puberty, had a relatively higher risk of showing deterioration of more than 30m compared to older patients. Our findings provide the first longitudinal data using the 6MWT in ambulant SMA patients.