In advanced hepatocellular carcinoma (HCC), allelic loss on chromosome 16q may occur. To better define the frequency of this alteration in small HCC and to more closely identify the affected region for further positional cloning of the putative tumor suppressor gene contained in this region, microsatellite polymorphism analysis was conducted on small, unifocal HCC, without signs of intrahepatic or systemic spread. We also tried to assess its possible correlation with hepatitis virus infections (HBV and HCV) and cellular proliferation rate. DNA from 35 small (<4 cm), unifocal HCC and from the corresponding nontumorous surrounding tissue was analyzed by 10 sets of microsatellite polymorphic markers. Serologic markers for hepatitis virus B and C infections were investigated in all cases. AgNOR protein quantity was assessed by image analysis on cryostatic sections stained with silver. The percentage of tumours with allelic imbalance ranged from 11.1 to 37%. The minimal involved region was assessed at 16q24.3, corresponding to the D16S413 marker, which was also the most commonly affected locus (10 of 27 informative cases, 37%). Allelic imbalance on chromosome 16q was significantly associated with HBV infection: 8 of 10 cases showed an actual or previous HBV infection in the group showing allelic imbalance, versus 6 with a previous HBV infection out of 25 in the control group (P < 0.01). No difference was found as far as HCV infection is concerned. The mean (+/-SE) AgNOR protein value in six cases showing allelic imbalance was 8.36 +/- 1.2 microm2, compared to 6.45 +/- 0.68 microm2 in 13 cases retaining both the alleles at 16q but the difference proved not statistically significant. In conclusion, in this series of small, unifocal HCC the minimal region of allelic imbalance on 16q was restricted to 16q24.3. It was found to be associated with HBV infection but not with increased cellular proliferation rate.