DNA aneuploidy in follicular thyroid neoplasia. The potential value of DNA aneuploidy, in distinguishing benign from malignant follicular thyroid neoplasms, was studied. The nuclear DNA content of 65 follicular thyroid neoplasms (52 adenomas and 13 carcinomas) was determined by flow cytometric analysis of paraffin embedded material; in 58 cases preparations were technically satisfactory. In 22 follicular neoplasms DNA analysis was also performed on fresh material obtained by fine needle aspiration of surgical specimens. Cell cycle analysis was performed on both fresh and fixed specimens. An aneuploid DNA profile was found on analysis of fixed tissue in eight of 45 (18 per cent) follicular adenomas and four of 13 (31 per cent) follicular carcinomas. DNA aneuploidy was also found in six of the 22 (27 per cent) fresh preparations from follicular adenomas. The frequency of DNA aneuploidy in apparently benign and malignant follicular neoplasms was similar. Follicular thyroid neoplasia are best regarded as a single entity with a low incidence of local and distant spread. All follicular neoplasia are therefore best excised. 