To evaluate the change of serum myoglobin in the thyroid disease, we measured serum myoglobin by radioimmunoassay in 24 normal subjects, 34 patients with hyperthyroidism (20 untreated, 14 treated) and 5 patients with hypothyroidism. The mean values of serum myoglobin in 24 normal control, 20 untreated hyperthyroidism, and 14 treated hyperthyroidism were $50.9{pm}17.0ng/ml,;39.1{pm}20.3ng/ml;and;51.0{pm}18.3ng/ml$ respectively. The serum myoglobin level of untreated hyperthyroidism seemed to be lower than that of normal control without statistical significance. The serum myoglobin levels in 5 patients with hypothyroidism ranged 28-156 ng/ml, and 3 patients showed increased serum myoglobin level. We observed one case with severe degree of thyrotoxic myopathy whose serum myoglobin level was markedly elevated initially and was decreased to normal range after treatment. This results suggests that the determination of serum myoglobin might be useful in the evaluation of thyrotoxic myopathy.