The nuclear DNA content of 67 follicular adenomas and 7 adenomatous goiters was determined by flow cytometry. The DNA analysis was done with the use of paraffin-embedded tissue, but in 14 cases of follicular adenoma fresh material obtained by fine needle aspiration biopsy was also available. An abnormal DNA stemline was found in 27% (18/67) of the adenomas and in 1 (14%) of the adenomatous goiters. Four (29%) of the 14 adenomas analyzed from fresh samples were aneuploid. In none of the 12 aneuploid or 40 diploid adenomas followed for more than 5 years (mean, 7.2 years) did distant metastases develop after surgical removal of the adenoma. The paraffin blocks of cases with DNA aneuploidy were sectioned subserially at 1-mm intervals, but none of these tumors showed invasion into the capsule or blood vessels. It is concluded that DNA aneuploidy is common in histologically benign follicular adenomas of the thyroid, and that DNA aneuploidy does not appear to be associated with adverse prognosis in follicular adenomas if the tumor is surgically removed.