A retrospective analysis of surgically resected thyroid nodules by stereology and DNA flow cytometry was performed in 15 follicular adenomas and 15 well-differentiated follicular carcinomas. The criteria for diagnosis were based on the WHO classification of thyroid tumours. By area-weighted random sampling of the visual fields for light-microscopic stereology, any subjective selection bias was precluded, and each point within the embedded neoplastic tissue was given equal probability of being analyzed. 150-250 tumour cell nucleus (TCN) profiles were studied per case by a semiautomatic image analyzing system. Flow cytometric analyses included measurement of the DNA-index, and the percentages of cells in S-phase and in G2/M-phase. Adenomas and carcinomas did not differ in stereological estimates related to TCN size. As examination of the stereological techniques by nested analysis of variance showed that this result cannot be ascribed to inaccurate methods, it follows that determination of TCN size is not a useful tool for the diagnosis of malignancy in well-differentiated thyroid tumours. Both groups included similar proportions of diploid and aneuploid neoplasms. In the carcinoma group the percentage of tumour cells in the G2/M-phase was more than twice as high than in the adenoma group (P less than 0.01). The ratio of short to long TCN profile axis was significantly smaller, and the coefficient of variation of TCN profile area was significantly higher in carcinomas than in adenomas. These findings are consistent with more unequiaxed TCN and higher anisokaryosis in the malignant tumours. Despite the significant differences, however, overlap of data from individual cases precludes the use of these estimates as diagnostic criteria. Pooling of the follicular tumours and dichotomizing the sample by the DNA-index showed that mean TCN profile area is increased and surface-to-volume ratio of TCN is decreased in aneuploid as compared to diploid tumours. This finding indicates that aneuploidy is associated with an increase of TCN size.