The initial muscle biopsies of 30 patients with active polymyositis or dermatomyositis have been assessed histologically and morphometrically. Only 1 patient (3.3%) had a normal biopsy, while 3 had a normal electromyograph and 8 had a normal serum CPK level. No significant morphological differences were found between the various diagnostic subgroups. Sequential biopsies of 11 of these patients treated either with corticosteroids only or with intensive immunosuppression were studied. With response to treatment atrophy factors, variability coefficients, internal nuclei count, and the various histological changes improved in the whole group of patients. However, there was no consistent correlation between any of the morphological features or between these and clinical parameters in individual patients. The percentage of fibres with internal nuclei in the initial biopsy was lower (P < 0.001) in patients responding to up to 60 mg prednisolone per day than in those who eventually required cytotoxic drugs in addition.