In Hodgkin's disease, cytogenetically aberrant clones have been demonstrated in a minority of cases studied. In the remaining cases, only normal metaphases have been found, but it is questionable whether normal karyotypes actually correspond to the pathognomonic Hodgkin and Reed-Sternberg (HRS) cells. Numerical aberrations could be studied by fluorescence in situ hybridization (FISH). However, in Hodgkin's disease, the percentage of tumor cells is mostly below the detection limit of FISH, which is near 1%. With the technique of simultaneous fluorescence immunophenotyping and interphase cytogenetic analysis (FICTION), this problem can be overcome. By FICTION, hybridization signals can selectively be evaluated within the CD30a+ cell population. We have studied 30 cytogenetically analyzed cases of Hodgkin's disease by means of FICTION. In all cases, we found numerical chromosome aberrations within the majority of CD30+ HRS cells. In cases with complex and hyperdiploid karyotypes, the cytogenetic results agreed with the FICTION data. There was considerable variability in the chromosome numbers, demonstrating that karyotype instability is an in vivo phenomenon of HRS cells. Lymphocytes never displayed numerical chromosome changes. Our results indicate that HRS cells regularly exhibit numerical chromosome aberrations and that the chromosome numbers are always in the hyperploid range.