Information about a tissue's proliferative activity can be obtained from the immunocytochemical investigation of proliferating cell nuclear antigen (PCNA), an auxiliary protein of DNA polymerase delta expressed by cycling cells. To determine whether a relationship exists between morphology and PCNA expression in normal, regenerative, and malignant neoplastic hepatocytes, this study was undertaken on 48 fine-needle aspiration cytology (FNAC) cell blocks from eight normal livers, eight cirrhotic livers, and 32 hepatocellular carcinomas (HCCs), as well as on 41 needle or wedge biopsy specimens from 10 normal livers, 13 cirrhotic livers, one focal nodular hyperplastic liver, and 17 HCCs. Anti-PCNA monoclonal antibody PC10 was applied to formalin-fixed, paraffin-embedded tissue using the avidin-biotin method. Proliferating cell nuclear antigen immunoreactivity was evaluated as follows: absent; minimal, less than 5% positive nuclei; grade 1, 5% to 25% positive nuclei; grade 2, 26% to 50% positive nuclei; grade 3, 51% to 75% positive nuclei; and grade 4, 76% to 100% positive nuclei. In both the FNAC and biopsy series normal and regenerative livers were either completely negative or minimally immunoreactive (under 5% positive nuclei). In contrast, all well-differentiated HCC cases exhibited over 15% positive nuclei. Most well-differentiated HCCs were grade 1 (85.7% in the FNAC series and 76.92% in the biopsy series) and the majority of moderately differentiated HCCs were grade 3 (63.63% in the FNAC series, but only 50% in the biopsy series). Therefore, absent or minimal PCNA immunoreactivity seems to be a useful adjuvant to discriminate normal/regenerative liver from HCC, whose degree of differentiation tends to correlate with the level of PCNA expression. These observations apply to both the FNAC and biopsy series, which yielded very similar data.