One hundred and seven patients with polymyositis or dermatomyositis were followed for an average of 5 years. In 100 patients it was possible to evaluate the effect of treatment with steroids or a combination of steroids and immunosuppressive drugs. Fifty patients improved, 50 did not. Compared with the treatment-resistant group the treatment-responsive patients were of younger age, did not have an associated malignancy or cardiac disease and began treatment within 24 months after the onset of muscular weakness. In the treatment-responsive group the degree of improvement was correlated to the mean dose of steroids given during the first 3 months of treatment. Eighty-seven per cent of surviving responsive patients had no or slight disability at the end of the investigation. The mortality rate was 23% and was highest in the treatment-resistant group. Comparison between different treatment programs was possible and based on the results, a treatment schedule could be recommended. The principles of this schedule are: During the first month high doses of prednisone or prednisolone should be given daily. From the second month, an alternate-day administration can be employed. Tapering to maintenance dose should be gradual and slow. The treatment period should be long. The mean duration in the present series was 27 months.