Eight muscle biopsies (3 from the left biceps, 3 from the left gastrocnemius, and 2 from the left quadriceps) of patients with hypothyroid myopathy were studied in the light of the previous literature. Some new observations have been made. First, the percentage of type II fibres is higher than that of type I fibres in all but 1 cases before treatment. Second, 5 of 8 cases before treatment disclosed 'core-like' structures, readily recognized with oxidative enzyme preparations in eccentric positions or in the periphery of type I fibres. They were reactive with myofibrillar adenosine triphosphatase (ATPase) activity and periodic acid-Schiff(PAS) staining, whereas with hematoxylineosin (HE) and modified Gomori trichrome the regions were more strongly reactive than the rest of the fibre. When examined by electron microscopy, within the 'core-like' structures of affected fibres, the A, I, and Z banding pattern was markedly disrupted. These structures disappeared after treatment with L-thyroxine. Third, none of the cases with Hoffmann's syndrome showed individual muscle fibre hypertrophy. Further study of these findings may yield information on clarifying the characteristics of muscle pathology of hypothyroid myopathy.