Clinical assessment for the presence of myopathy was performed in fifteen untreated hypothyroid patients. In many of these patients plasma creatine kinase (CK) activities, tests of skeletal muscle function and vastus lateralis needle biopsies were performed. Muscle symptoms were extremely common, occurring in thirteen patients. Proximal muscle weakness, preferentially affecting the legs, muscle pains or cramps and delayed tendon jerk relaxation represented the commonest features of hypothyroid myopathy. Detailed descriptions are given of patients with muscle hypertrophy, muscle atrophy and developmental failure to illustrate the wide clinical spectrum. If examination revealed signs of myopathy, in particular if there was delayed relaxation of tendon jerks, biochemically severe hypothyroidism (serum thyroxine less than 20 nmol/l) was likely (P less than 0.01). The latter was a frequent finding in six patients with serous effusions (P less than 0.01). This subgroup also had a mean plasma CK activity of 1339 IU/l compared to 679 IU/l (normal 10-120 IU/l) for the whole hypothyroid group, and all five patients in this subgroup in whom vastus lateralis needle biopsies were performed showed abnormalities. Quadriceps force measurements and ankle jerk relaxation times were more sensitive than clinical examination in detecting muscle weakness and delayed relaxation. A reduced maximum relation rate (MRR) of the quadriceps, vastus lateralis needle biopsy abnormalities, and myopathic features on electromyography were characteristic.