The dimensions of myonuclei and satellite cell nuclei in Duchenne's muscular dystrophy (DMD), polymyositis, and normal controls were compared in order to determine whether differences in satellite cell populations (previously reported by these and other authors) could be attributable to changes in the relative dimensions, thereby biasing the counts. The nuclei were measured directly from semithin resin sections using computerized measureing techniques, thereby avoiding errors due to photographic enlargement. In both the control and polymyositic groups, the satellite cell nuclei (8.30 μm and 8.81 μm respectively) were significantly shorter than the myonuclei (11.75 μ and 13.00 μm). Dystrophic myonuclei (10.98 μm) were significantly shorter than polymyositic myonuclei, but dystrophic satellite cell nuclei (11.62 μm) were significantly longer than both polymyositic and normal control satellite cell nuclei. The mean nuclear area in transverse sections was significantly greater in both myopathies than in the control material for both myonuclei and satellite cell nuclei. Myopathies were significantly larger than satellite cell nuclei in all groups. When the values for the lengths of the nuclei were used to adjust previous estimates of satellite cell populations, it was found that earlier conclusions were still valid, i.e., that there is a significant increase in the number of satellite cells in the dystrophic muscle fibre population.