The nuclear DNA contents of 54 hepatocellular carcinomas were measured on cytologic smears using a scanning microdensitometer at a wavelength of 550 nm. The cellular materials were aspirated under ultrasonographic guidance and stained with modified Feulgen method. The DNA modal values, which could be defined in 51 cases, ranged from 2.2 to 9.8 ploidy, with 46.3% (25/54) in hypertetraploidy. The hepatocellular carcinomas in patients with higher serum levels of alpha-fetoprotein contained more hyperploid cells. The DNA modal values did not correlate with the age, HBsAg status, tumor size and histological grading of hepatocellular carcinoma. The S-phase fraction values, which could be estimated on the DNA histograms in 43 cases (79.6%), ranged from 1 to 35 (mean +/- S.D. = 10.6 +/- 8.1). The S-phase fraction values were higher in high grade hepatocellular carcinomas (13.4 +/- 8.5) than low grade hepatocellular carcinomas (6.1 +/- 4.4), p less than 0.01. The mean S-phase fraction values were also significantly higher in hepatocellular carcinomas with alpha-fetoprotein levels of more than 100 ng per ml than those of less than 100 ng per ml (12.9 +/- 8.0 and 7.8 +/- 5.6, respectively). The S-phase fraction values also did not correlate with the age, HBsAg status and tumor size.