Between October 1984 and June 1987 in 45 patients (9 males, 36 females) pre- and postoperatively the immunocompetent cells in the peripheral blood were measured using OKT- and LEU-11-antigens. 20 patients suffered from malignant and 25 patients from benign diseases. For statistical analysis, H-test according to Kruskal and Wallis was used. Preoperatively, the total account of T-lymphocytes was decreased in patients with thyroid carcinomas, but showed no differences compound to patients with benign diseases postoperatively. No differences were observed in lymphocytic subpopulations in all groups. Parallely, in 50 patients (14 males, 36 females) immunocompetent cells were counted in the resected specimens. There were investigated 29 carcinomas and 21 benign thyroid lesions using the LEU-antigens. Between the group with malignant diseases and that with benigne lesions of the thyroid, no statistically significant differences were observed.