A common measure of activity limitations for both children and adults with neuromuscular disorders was developed using the Rasch model. A self-reported questionnaire containing daily activities was submitted to 245 adult patients and to the parents of 124 affected children from the two major Belgian communities. They were asked to provide their perceived difficulty in performing daily activities on a three-level scale. The 22 items of the final scale define a unidimensional and linear measure of activity limitations and show a continuous progression in their difficulty. The item difficulty hierarchy is invariant with regard to the diagnosis, community, gender and age. The scale exhibits a good precision, since the 22 items are well targeted on our sample (r=0.96); furthermore, it is reproducible over time (ICC=0.93). The patients' measures are related to the Functional Independence Measure motor score (rho=0.85), to the Brooke (rho=-0.63) grade and to the Vignos (rho=-0.83) grade.