Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal-dominant disorder localized to 4q35. Neither the gene nor the gene product has been identified. There is presently no established treatment for FSHD. Prospective data on the natural history of this disorder are essential for the effective design of therapeutic trials. We systematically followed 81 well defined FSHD patients for up to 3 years using a standardized protocol that included manual muscle testing (MMT), maximum voluntary isometric contraction testing (MVICT), and functional testing. Muscle strength was strongly associated with measures of muscle mass, age at onset, and duration of disease. Decline in strength over time was slow but detectable with both MVICT and MMT. The magnitude of decline was not associated with either age, gender, age at onset, or duration of disease. This study establishes reliable and valid measures of disease state and progression for use as outcome variables in clinical trials in FSHD, and also provides guidelines for determining sample size and duration of follow-up. A two-armed clinical trial involving 160 patients per group and 1 year of follow-up would provide 80% power to detect complete arrest of the progression of the disease. Trials with fewer patients would thus have adequate power to detect only improvements in strength, unless follow-up duration were extended well beyond 1 year.