Previous reports on the analysis of the human monoblastic cell line U937 had described several sublines containing unidentified rearrangements and marker chromosomes. In order to determine the true nature of the rearrangements, conventional banding analysis was carried out with various combinations of molecular cytogenetic techniques: comparative genomic hybridization, fluorescence in situ hybridization (FISH) with whole chromosome painting probes, and microdissection and reverse painting FISH. The origins of the marker chromosomes were identified and the composite karyotype is described.