2521 patients of the Lódź Outpatient Endocrinological Clinic (2290 females, 231 males; inhabitants of the central region of Poland Lódź City, Lódź Metropolitan Area, Piotrków, Płock, Sieradz, Skierniewice and Włocławek Provinces in which committed dose equivalent to the thyroid was between 2.7-7.0 mSv [min.-max.] in Skierniewice Province and 4.6-11.7 mSv in Płock Province) were included in the study. The patients were divided into 5 groups: I--persons who did not take the protective dose of potassium iodide (KI) after Chernobyl Nuclear Power Plant accident and did not received any treatment with thyroid preparations or hormones at that time (n = 1282), II--patients who receive KI, once or several times (n = 774), III--patients who took orally iodine tincture or other iodine-containing preparations for the above purposes (n-37), IV--patients who took tablets of Thyroideum (Polfa) Thyroideum siccum (dry thyroid extract), once or several times, as a prophylactic action (n = 79), V--patients who were in the course of continuous treatment with Thyreoideum or thyroid hormones at the time of Chernobyl accident (n = 349). The analysis was performed for all the patients jointly, as well as separately for: either sex, three age groups (18-30, 31-55, 56-70 yrs) and 7 administrative areas specified above. All the patients were subjected into complex clinical examination, serum TSH, T3, T4 concentrations, anti-thyroid membrane antibodies (ATMA) and antithyroglobulin antibodies (ATg) titres, as well as ultrasound, scintigraphy, and fine needle aspiration biopsy of the thyroid (the last two according to indications) included. The patients were also examined by means of a special questionnaire (Patient's Inquiry Sheet), which was subsequently submitted to computer analysis. All the doctors' diagnoses from 1986 (17 different diagnoses) and 1990 (27 different diagnoses), as well as the course of diseases, were verified with use of a specially prepared IBM PC/AT computer program ChernStat.(ABSTRACT TRUNCATED AT 400 WORDS)