Vesicular changes in the myopathies of AIDS. Ultrastructural observations and their relationship to zidovudine treatment. Six patients with AIDS and AIDS related complex (ARC) who developed neuromuscular symptoms associated with vesicular changes in muscle fibres are reported. Two patients in the advanced stages of AIDS, who did not receive zidovudine, developed proximal limb weakness and wasting: both had a necrotising myopathy with an unusual segmental vesicular change of myofibres. There were numerous vesicles 0.1 to 2 microns in diameter produced by dilatations of the sarcoplasmic reticulum in fibres depleted of myofibrils. Four patients developed a myopathy while receiving zidovudine for AIDS. One of these had an inflammatory myopathy which showed the development of vesicular change due to enlargement and electron lucency of mitochondria. The three other patients with ARC developed muscle pains or weakness and elevated serum CK while on zidovudine. These patients also showed vesicular changes due to enlargement and electron lucency of mitochondria associated with disruption of sarcomeres and the presence of cytoplasmic bodies. The muscular symptoms resolved when ziduvodine was stopped and repeat biopsy in one case revealed no abnormalities. 