The aim of this work was to assess caspase-3 activity in the tissue of papillary thyroid carcinomas of patients and analyze the peculiarities of changes in this activity depending upon a number of pathomorphological and clinical features of tumoral process.Caspase-3 activity was determined by spectrophotometry with regard to acetyl-asp-glu-val-asp-paranitroanilide.At initial stages of tumor development, in the absence of metastases to lymph nodes, blood and lymphatic vessel invasion by tumor cells, extrathyroid spreading of tumor, sclerotic and fibrous changes in tumor stroma, and in the presence of tumor capsule, caspase-3 activity in papillary carcinoma tissue was higher compared to unchanged thyroid tissue of normofollicular structure. In case of a more aggressive behaviour of tumor, enzyme activity in carcinoma tissue did not differ significantly or (in case of extrathyroid spreading of tumor) was decreased compared to that in extratumoral tissue. In combination, this was expressed by a progressive decrease in caspase-3 activity in tumor tissue with increasing T category. Сaspase 3 activity was found to be much higher in the tissue of papillary carcinomas of follicular-papillary structure and lower in the tissue of tumors of mixed structure with solid areas, compared to that in the tissue of papillary carcinomas of typical papillary structure.The data obtainеd in assessing caspase-3 activity suggest that the intensity of spontaneous apoptosis of human papillary thyroid carcinoma cells depends upon the stage and aggressiveness of tumoral process.