In order to determine the incidence and to understand recent trend of thyroid neoplasm and other thyroid diseases among Korean, a nation-wide collection of biopsy-confirmed and surgically removed thyroid lesions from 30 pathology laboratories of university and general hospitals was made over a 3-year-period from 1986 to 1988. These tumors and tumor-like lesions of the thyroid were classified according to the WHO classification. Results of this study were compared with those of the previous in Korea and other countries. Among the 7758 collected cases from 7449 patients, adenomatous goiter was the most common, comprising 2681 cases (34.6%), followed by follicular adenoma, 1868 cases (24.1%) and papillary carcinoma, 1474 cases (19%). Neoplastic condition comprised 48.8% (3786 cases). Of malignant tumors, papillary carcinoma was the most frequent accounting for 79.8%. The female to male ratio was 7.5:1 with female predominance. This female predominance was noted in all but two thyroid diseases. Medullary carcinoma showed equal distribution in both sexes, and two cases of malignant lymphoma developed in males. Thyroid diseases were common at the 3rd to 6th decades with peak incidence at the 4th decade. The right lobe of the thyroid was more frequently involved than the left lobe (1.6:1). The diagnoses were made largely on the surgically excised specimen (85.3%). Multiplicity of the lesions revealed single lesion in 64%, multiple and diffuse lesion in 36% of cases investigated. Average size of the lesion was in the range of 1-5 cm in diameter (62%).