OBJECTIVE Dipeptidyl aminopeptidase IV (DPP IV) activity in cytological samples from a follicular thyroid tumor is the most sensitive and specific indicator for the detection of follicular carcinoma of the thyroid gland. Dipeptidyl aminopeptidase IV activity is independent of cytological characteristics and superior to other clinical findings. DESIGN AND PATIENT SELECTION: Among the patients surgically treated for follicular thyroid tumors, we recruited approximately equal numbers of those with true-positive (n = 19), true-negative (n = 26), false-negative (n = 16), and false-positive (n = 18) cytological characteristics. METHODS We examined DPP IV activity using cytological specimens obtained from 35 patients with follicular thyroid carcinomas and 44 patients with follicular adenomas. Tumor size, patient age, serum thyroglobulin level, and ultrasonographic findings were also analyzed. RESULTS The positive rate of DPP IV activity was 97% in 35 patients with follicular thyroid carcinomas and 5% in 44 patients with follicular adenomas, resulting in a sensitivity of 97%, a specificity of 95%, and an overall accuracy of 96%. This discriminating ability of DPP IV activity was far higher than that of tumor size, patient age, serum thyroglobulin level, or ultrasonographic findings. CONCLUSIONS Positive DPP IV activity in cytological samples is the best discriminatory marker between follicular thyroid carcinoma and follicular adenoma. Its application could alter the clinical management of patients with follicular thyroid tumors.