To assess metabolic turnover in patients with hyperthyroidism weight loss in mg . kg-1 . min-1 (specific weight loss, SWL) was measured by means of a new type of scale working on the buoyance principle with a minimal susceptibility of 10 mg per 100 kg. Weight loss rate was significantly increased in hyperthyroid patients (n=22;SWL, 22.0 +/-1.4 (SE) mg. kg-1 . min-1) compared to health controls (n=11; SWL, 10.0 +/-0.4 (SE) mg . kg-1 . min-1). The fall in SWL paralleled the decrease in thyroid hormone levels in serum in the beginning of antithyroid treatment and remained unchanged thereafter in spite of a further drop in thyroxine (T4) and triiodothyronine (T3) levels. After six weeks of treatment SWL reached the upper normal range despite hormone levels in the low range of normal. The findings indicate that the normalization of the basal metabolic rate as assessed by the estimation of SWL is slower than the decline in serum hormone levels. There was no correlation between thyroid hormone levels and SWL before initiating therapy or during antithyroid treatment (T4 vs. SWL, r = 0.22;n = 56). Thus serum levels of T4 and T3 appear to be rather insufficient indices regarding the respective metabolic state as estimated by SWL.