Erythrocyte Ca2+-ATPase activity was measured in membrane ghosts from hyperthyroid and hypothyroid patients and compared with that in normal subjects. Basal enzyme activity was significantly increased in the hyperthyroid group and decreased in the hypothyroid group. In vitro responsiveness of the enzyme to calmodulin, the activator protein for Ca2+-ATPase, and to T4 was decreased in both hyper- and hypothyroid ghosts. Although the membrane content of endogenous calmodulin was low in hypothyroidism, this is an unlikely explanation for the low basal activity of Ca2+-ATPase. A limited number of patients from each group were restudied when they were euthyroid after 8 or more months of treatment. Basal, T4-stimulated, and calmodulin-stimulated Ca2+-ATPase activities returned to normal, with the exception of calmodulin responsiveness in the membranes from previously hypothyroid patients. This decreased responsiveness may have been a reflection of either increased patient age or underlying chronic (non-thyroidal) illness. This study provides clinical confirmation of previously reported in vitro studies suggesting that the set-point of Ca2+-ATPase activity is, in part, a function of endogenous thyroid hormone levels.