Autoimmune postpartum thyroiditis (PPT) has been thought of as one of the organ-specific autoimmune diseases. The present study was designed to investigate whether the immunological changes during the postpartum period might induce this disease, by comparing the circulating lymphocyte subsets and antibody-dependent cell-mediated cytotoxicity (ADCC) between normal postpartum women and PPT patients. The results were as follows: 1) No significant differences in the circulating total T lymphocyte population, or suppressor T lymphocyte subsets, or in Th/Ts ratio were found among 25 PPT patients, 11 normal postpartum women and 11 normal non-pregnant women. 2) In PPT patients, helper T lymphocyte subsets were fewer in proportion than those of normal postpartum or non-pregnant women. However, B lymphocyte population (19.7±7.8%) and ADCC activity (.41±13) in PPT patients were comparable to those in normal postpartum women (18.3±4.8%, .42±.11), although they were significantly greater than those in normal normal non-pregnant women (13.3±5.9%, .29±.07).