We have analysed the nuclear DNA content in a series of 33 thyroid tumours (18 carcinomas and 15 adenomas) and, for comparison, in 189 nodular strumas and 17 tissue samples of normal thyroids, calculating cell cycle parameters and presence of aneuploid DNA. Cell suspensions were prepared for each of three tissue fragments taken for different areas of the surgical specimens. Cell subpopulations with aneuploid DNA content were present in 39% of carcinomas, 27% of adenomas, 20% of strumas, but were absent in the samples from normal thyroids. The presence of hyperdiploid DNA was much more frequent than that of hypodiploid DNA in all pathologic tissues. The mean proliferation index was 14.1 in the carcinomas, 6.6 in the adenomas, 7.1 in the strumas and 6.1 in normal thyroid tissues. The percent of histograms with a coefficient of variation of the G0-G1 peak greater than 5 was highest in the carcinomas (26%). The significance and implications of the reported data are discussed for the interpretation of thyroid neoplastic pathologies.