One hundred and sixty-two patients with Duchenne muscular dystrophy (DMD) were followed over a 10-yr period to provide a profile of impairment and disability. The median height and weight of DMD boys were normally distributed before ages 9-10, but during the second decade height was markedly reduced, and weight was no longer normally distributed. Younger boys gained more weight than normals, whereas older individuals actually showed weight loss. Manual muscle test (MMT) measurements showed loss of strength in a fairly linear fashion from ages 5-13 yr, -0.25 MMT units per year. Upper extremity muscles were stronger than lower extremity muscles, proximal muscle groups were weaker than distal muscle groups, and extensor muscles were weaker than flexor muscles. There was no side dominance. There was a change in the rate of strength loss at 14-15 yr, and the decline slowed to only -0.06 MMT units per year. Although MMT and quantitative strength measurement profiles were similar, the latter were far more sensitive. In general, by the time strength declined to MMT grade 4, isometrically measured strength was 40-50% of normal control values. Joint contractures were rare before age 9, increased in frequency and severity with age, and were present in most individuals older than 13. Lower extremity contractures were strongly related to onset of wheelchair reliance, but there was no association between muscle imbalance around a joint. The prevalence of scoliosis increased between ages 11 and 16, with about 50% of the boys acquiring scoliosis between ages 12 and 15, corresponding to the onset of the adolescent growth spurt. Wheelchair reliance and scoliosis were both age-related. Percent predicted forced vital capacity declined at different yearly rates: ages 7-10, -0.3%; ages 10-20, -8.5%; after age 20, -6.2%. There was a direct relationship between percent predicted FVC and MMT scores. Decreased airway pressures, especially maximal expiratory pressure, appeared earlier than reductions in FVC but followed the same pattern. Thirty percent of the DMD boys had a history of respiratory complications, and the frequency increased with age. Spine deformity did not have a significant additive effect on the age-related decrement in pulmonary function. There was a high occurrence (79%) of abnormal electrocardiograms with age-related progression of some abnormalities, but only 30% of the patients had a history of cardiovascular complications. Functional level grades and timed motor performance measurements had a nonlinear relationship with strength and age.(ABSTRACT TRUNCATED AT 400 WORDS)