The clinical and myopathological features of 13 patients with a myopathy occurring in association with hypothyroidism are presented. Seven patients had hypothyroid myopathy, including two with the Hoffmann syndrome and one with the Kocher-Debre-Semelaigne syndrome. Five patients had an inflammatory myopathy and one had polymyalgia rheumatica. Serum CK activity was elevated up to 12-fold in the patients with hypothyroid myopathy and returned to normal after treatment with thyroxine. Pathological changes in these cases included type 1 or type 2 fibre atrophy or hypertrophy, myofibre necrosis and regeneration in four cases and, in one case, prominent core-like areas containing amorphous granulo-filamentous material. The findings in this series of cases illustrate the clinical and histopathological heterogeneity of patients with hypothyroid myopathy and the need to consider other myopathies in hypothyroid patients who present with muscular symptoms.