Data were collected prospectively over a 10-yr period from 53 subjects with facioscapulohumeral muscular dystrophy (FSHD) to provide a profile of impairment and disability. Manual muscle testing (MMT) indicated greater involvement of proximal musculature, although a subgroup demonstrated early weakness of the ankle dorsiflexors. Asymmetry of upper extremity musculature was noted, with greater weakness of selected dominant limb muscle groups. Weakness, in general, was relatively mild, with an overall mean MMT score of 3.7 units. The rate of strength loss was quite slowly progressive, a decline of only -0.22 MMT units per decade of age. An early age of onset was associated with greater likelihood of more severe and progressive weakness. Isometric and isokinetic quantitative strength testing revealed that all muscle groups were 36-68% weaker than a control population. Although nearly 50% of the subjects had vital capacity evidence of restrictive lung disease, only 13% had severe involvement, and only 22% had a history of pulmonary complications. There was no age or disease duration effect on pulmonary function measurements or complications. As with the other neuromuscular diseases, maximal expiratory pressure measurements were more sensitive than other pulmonary function tests. Abnormal electrocardiogram findings were rare and minor and not related to overt cardiac disease. Contractures were rare and mild. Thirty-five percent of the patients had spine deformity; however, most had hyperlordosis. Intellectual function was normal, and there were few abnormalities on personality tests. Functional testing demonstrated wide variation in disability with FSHD, but motor weakness uniformly translated into impaired motor performance skills. This profile demonstrates the clinical heterogeneity of FSHD.