Twelve patients with subacute thyroiditis were divided into two groups and treated with prednisolone or salicylate. The initially elevated T4, T3, free T4 (FT4), free T3 (FT3) and erythrocyte sedimentation rate (ESR) were reduced during the early phase within about 4 weeks in both groups. The serum levels of thyroglobulin (Tg) were elevated in both groups treated with salicylate and prednisolone (252 +/- SD 117 ng/ml and 233 +/- SD 157 ng/ml, respectively) at initial examination. The serum level of Tg declined during the early phase with prednisolone treatment, and it reached normal values at the end of the early phase (17 +/- SD 15 ng/ml). With salicylate treatment, the decline of levels of Tg was delayed and it was elevated (80 +/- SD 34 ng/ml) despite normal levels of thyroid hormones and ESR at the end of early phase. The serum level of Tg at the end of the early phase of prednisolone treated was significantly lower than that of salicylate treatment (P less than 0.01). It is suggested that the effect of prednisolone on rapid decrease of Tg may be related to its inhibitory action of intrathyroid hydrolysis.