In a large, mainly outpatient, series of hyperthyroid patients who attended a district general hospital the serum concentrations of calcium and albumin were measured before and in many cases after treatment. The calcium level (mean +/- SD) before treatment (2.41 +/- 0.21 mmol/l, n = 437) was significantly higher (P less than 0.01) than afterwards (2.36 +/- 0.15 mmol/l, n = 232) and the albumin level rose when the patients became euthyroid (from 40.5 +/- 3.1 g/l to 44.0 +/- 2.4 g/l; P less than 0.01). After treatment neither value differed from those of an unselected group of out-patients. The usual relation between the serum concentrations of calcium and albumin did not hold in the hyperthyroid subjects but reverted to normal on treatment; the variation, probably due to an increase in ionized calcium, leads to an overestimate of the 'corrected calcium' when conventional methods are used to calculate this figure. Thus, using a conventional formula 8.5% of our hyperthyroid patients would appear to have a calcium greater than 2.65 mmol/l (normal mean plus 2 standard deviations) whereas using a correction factor specific for the hyperthyroid situation the figure is reduced to 5.7% which is only twice the expected proportion. The calcium level was significantly greater (P less than 0.001) in those patients in whom initial T3 concentration was high (greater than 7.2 nmol/l). There was no effect of T4 upon serum calcium which could not be accounted for by the action of T3. In this series of 437 patients there was no case of symptomatic hypercalcaemia. The maximum value was 2.80 mmol/l in a patient with coincident primary hyperparathyroidism. Significant hypercalcaemia is rare in hyperthyroidism.