We have investigated 67 patients with proven Becker muscular dystrophy (BMD) using a standard protocol including a detailed history and a functional and clinical examination. Our aim was to define the natural history of the disease in a large cohort of patients in the light of the diagnostic methods now available. In all patients with or without an X-linked family history, the diagnosis was confirmed by the identification of a deletion or other abnormality in the dystrophin gene, and abnormal dystrophin on immunoblotting and immunocytochemistry of muscle biopsy samples. In graphs of functional and muscle score against age, two groups of patients emerged. In the larger group the disease was milder and patients remained ambulant into their forties or beyond. A smaller group had more severe disease with a slightly earlier onset, much earlier loss of ambulation, more frequent abnormal electrocardiographic findings and much lower reproductive fitness. The relationship of these clinical findings to the genetic and protein abnormalities found in the patients is explored in the accompanying paper.