To gain more insight into the role of chromosomal instability (CIN), the cytogenetic hallmark of most solid tumors, we performed fluorescence in situ hybridization (FISH) on interphase nuclei of cytological specimens enabling the correct detection of chromosome copies in intact tumor cells of 18 well (G1), moderately (G2), or poorly (G3) differentiated hepatocellular carcinomas (HCCs). A close correlation between the morphological dedifferentiation and increasing copy numbers and variation of FISH signals was seen for chromosomes 1 and 8, respectively (P < or = 0.0002). Four HCC G1 had constant chromosome patterns for chromosomes 1 and/or 8 with a mean of signals per nucleus < or =5.08 and < or =3 different signal combinations, indicating a low level of CIN, as confirmed by FISH using probes for centromeres of chromosomes 3, 7, and 17. In contrast to this, five HCC G2-3 revealed > or =8.46 signals per nucleus and 23-41 different signal combinations, indicating high levels of CIN. In the remaining cases, signal counts from 5.96-8.46 and 7-15 combinations were seen. Here, nuclei with constant aberration patterns and low copy numbers occurred alongside nuclei with inconstant patterns and high copy numbers. It is evident that in these cases a transition from well to moderately differentiated HCC developed in parallel to an increase in CIN, possibly induced by a major dysregulation of mitotic control mechanisms. In conclusion, CIN may induce a stepwise increase of aneuploidy in HCC that is mirrored by the morphological dedifferentiation of tumor cells.