Using comparative genomic hybridisation, DNA copy number changes were investigated in 15 cases of pigmented villonodular synovitis of the knee joint. Additionally DNA content was analysed by flow cytometry. Screening revealed numerical chromosomal imbalances in five of the examined cases. A total number of 18 gains were detected. The most frequent gains involved subregions of chromosomal arms 22q and 16p and 16q. No losses were found. One of the cases showed an aneuploid DNA-pattern, which actually proved to be the case with the most numerical chromosomal changes.