Alkaline phosphatase (ALP; EC 3.1.3.1) isoenzymes were measured in the plasma of 63 untreated hyperthyroid patients (the hyperthyroid group), 58 treated hyperthyroid patients, and 100 blood donors. Total, liver, and bone ALP activities were significantly higher in the hyperthyroid group than in the treated hyperthyroid group or the blood donors. Bone ALP was more frequently and more markedly abnormal than liver ALP. Intestinal ALP did not differ significantly between the groups. The hyperthyroid patient group had significantly higher plasma calcium concentrations and lower serum parathyrin concentrations than those of the treated hyperthyroid group.