Follicular neoplasms of the thyroid in men older than 50 years of age. A DNA flow cytometric study The clinical behavior of follicular neoplasms of the thyroid in elderly men can be difficult to predict on histologic grounds alone. To assess the usefulness of DNA flow cytometry in predicting the metastatic potential of these tumors, the authors studied 44 primary and metastatic follicular neoplasms of the thyroid by DNA flow cytometry of paraffin-embedded tissue. The neoplasms were obtained from 44 men ranging in age from 50 to 79 years (mean, 60). There were 29 follicular adenomas, 11 primary follicular carcinomas (neoplasms with capsular and/or vascular invasion), and 4 metastatic follicular carcinomas. Follow-up information was available on 40 of the 44 patients. The mean follow-up was 114 months. Twenty-five of the 29 follicular adenomas had a diploid DNA content, 2 (7%) were tetraploid, and the DNA histograms on 2 were not interpretable. All patients with follicular adenomas had no evidence of disease (NED) at last follow-up. Eight of the 11 primary follicular carcinomas were diploid. Six of these patients had NED, one died with carcinoma at 82 months, and no follow-up was available on one. Three (27%) of the primary follicular carcinomas were aneuploid or tetraploid. Two of these patients had NED, and the third died with carcinoma 84 months after diagnosis. Two of the four metastatic follicular carcinomas were diploid and two (50%) were aneuploid or tetraploid. One of the two patients with diploid metastatic follicular carcinomas died with carcinoma, as did one of the two patients with aneuploid metastatic follicular carcinomas. These results suggest the following: (1) follicular carcinomas are more likely to be aneuploid or tetraploid than are follicular adenomas; (2) follicular neoplasms without capsular or vascular invasion may include a small number of aneuploid or tetraploid tumors; and (3) DNA ploidy does not add to the prognostic value of histologic studies alone. 