Despite difficulties in differential diagnosis, aspiration cytology offers unique advantages in the preoperative diagnosis of Hürthle cell (HC) tumors of the thyroid gland over immunocytochemistry and micromorphometry, which have not achieved the anticipated success in the diagnosis of HC tumors. Aspiration cytology is comparable, in terms of speed and accuracy, to microspectrometry of DNA content in HC tumors, a technique that is not convenient for routine use. Detailed morphologic analysis of 19 characteristics of the smears and of the cells revealed five parameters adding to the sensitivity, specificity and reliability of routine diagnosis: HC collection in the form of nests, marked or at least moderate anisocytosis and anisokaryosis, multinucleation and emphasized nucleoli. All five criteria are found in > 70% of HC tumors, and the rate of false positives is < 50%. The simultaneous presence of three or four of these five parameters occurred in 70-90% of HC tumors and would lead to a false-positive diagnosis of HC tumors in only 30% of cases. Routine cytologic diagnosis of HC tumors of the thyroid gland may be improved by adoption of these criteria.