Morphometric analysis of 23 follicular tumors (13 adenomas and 10 carcinomas) and 10 metastatic lesions from follicular thyroid carcinomas resulted in the identification of 5 significant parameters between the carcinoma and adenoma groups. With a classification rule using these features, the probability of a histologic diagnosis of adenoma vs. carcinoma was computed for each case. Using threshold values of probabilities of 0.25 and 0.75, only 1 of the 33 cases was not correctly classified on the cytologic smear alone; the remaining 32 cases were correctly classified. Morphometric analysis in follicular tumors of the thyroid may be of great help in individual patient care if used as a selective method by a well-trained cytopathologist.