We present the first comprehensive flow cytometric analysis of papillary carcinoma of the human thyroid. All but one of the 22 papillary carcinomas showed a unimodal distribution of the nuclear DNA content. The mean modal DNA was significantly lower in papillary carcinomas than in normal thyroid tissue, a finding which is contradictory to some earlier reports based on conventional cytophotometry. Tumors predominantly composed of Hürthle cells had a significantly higher mean modal DNA, whereas no significant difference was found between tumors with many ground glass nuclei and those lacking this nuclear feature. A very low percentage of S-phase cells was found both in the primary tumors and their metastases. This percentage was even lower than in normal thyroid tissue. The low proliferative activity may in part explain why most papillary thyroid carcinomas run such a surprisingly indolent course even after the establishment of regional lymph node metastases.