OBJECTIVE To analyze the diagnostic problems with fine needle aspiration biopsy in anaplastic thyroid carcinoma (ATC) and to describe the cytomorphologic characteristics in 113 cases. METHODS A retrospective analysis of 113 fine needle aspirates and 67 surgical specimens from 113 patients with ATC admitted to the Institute of Oncology, Ljubljana, in 1972-1992. RESULTS In a series of 113 fine needle aspirates of ATC, 3 (2.7%) were inadequate, 3 (2.7%) suboptimal and 107 (94.7%) diagnostic of malignancy. On reexamination, 96/107 (89.7%) were diagnosed as ATC, 6 (5.6%) as differentiated thyroid carcinoma, and 5 (4.6%) as a malignant tumor not otherwise specified. As to the predominant cell population, fine needle aspirates showed three different cell patterns: (1) pleomorphic cell (43 cases), (2) round cell (33 cases), and (3) spindle cell pattern (7 cases). In the present retrospective analysis we identified three main reasons for inadequate or nonrepresentative fine needle aspiration biopsy sampling: (1) tumor regressive changes (necrosis, hemorrhage, leukocytic infiltration), (2) extensive tumor fibrosis, and (3) distinct differentiated and anaplastic patterns in the same tumor. CONCLUSIONS The major diagnostic problem with fine needle aspiration biopsy (FNAB) of ATC is related to sample quality. Cytomorphologic features of ATC are highly specific and easy to recognize. Due to the simple technique and high diagnostic accuracy, FNAB is the method of choice in patients with ATC.