The importance of lightmicroscopical and immunohistochemical features of 38 recurrent differentiated thyroid carcinomas (27 papillary carcinomas (PC), 11 follicular carcinomas (FC] for post-operative serum thyroglobulin (TG) concentrations was analysed in regard to pre-operative serum TG levels with tumour type, histological and cytological differentiation, volume fraction of TG synthesizing tumour cells (TG immunohistomorphometry), tumour volume and radioiodine uptake (RIU). Serum TG concentrations increased with tumour size and the number of TG synthesizing tumour cells (r = 0.5). PC and FC did not differ in their volume proportions of TG synthesizing tumour cells, while TG serum levels in FC significantly exceeded those of PC of similar size. The low TG serum levels found in PC might be explained by a specific defect in thyroglobulin secretion. Carcinomas with partial or total cytologic metaplasia (e.g. oxyphilic carcinomas) had low volume proportions of TG synthesizing cells and low serum TG levels. Thirteen of the 38 differentiated carcinomas (34.2%) showed both high TG serum levels and positive RIU, 17 (44.7%) disclosed only elevated TG serum levels and 6 (15.8%) a positive RIU. In two cases (5.3%) TG serum levels were not elevated and RIU's were negative. TG immunostaining was positive in all 38 cases. In summary, TG serum levels depend on the following morphologic factors in differentiated thyroid carcinomas: 1) Number of TG synthesizing tumour cells, 2) Mode of TG secretion and 3) Cytological differentiation of the tumour cells. Serum TG levels did not predict total body iodine scan.