The association between mitotic rate and biologic behavior was studied in 127 cases of thyroid cancer. Based on the observation of 12,700 high-power fields (HPF), the mitotic rate varied from 0 to 316 (33.3 +/- 4.9) mitoses/100 HPF. Mitotic rate differed among the four types of thyroid cancers and correlated inversely with the differentiation of the tumors, being highest (156.8 +/- 32.9) in the undifferentiated tumors and lowest (17.3 +/- 2.4) in the papillary tumors. By analysis of variance (AOVA), the relations between mitotic rate and the sex or age of the patient, maximal diameter of the tumor, and vascular or capsular invasion were assessed, but significant relations were not found (r2 = 44.2 per cent). Follow-up observation, possible in 74 patients, showed mitotic rate to be significantly related to the survival period (Student's t-test; P values ranging from 0.02 to 0.05). The patients who were alive five and ten years after surgery had had lower mitotic rates than those who had died during the same follow-up period. The correlation of low mitotic rate with a high degree of differentiation and low potential for invasion might, in part, explain the better surgical cure rates for papillary thyroid carcinoma than for other types of thyroid cancers.