Chromosome studies were done on 56 patients with multiple myeloma to investigate their chromosomal abnormalities and to evaluate their clinical significance. Abnormal karyotype was found in 25 patients (44.6%), no relationship was seen between the frequency of chromosomal aberrations and age, sex, immunoglobulin isotype, bone marrow plasmocyte number and previous cytostatic therapy. Hyperdiploid and hypodiploid karyotypes were present in 10 and 13 patients, the chromosomes 1, 11 and 14 most often participated in structural rearrangements. Good relationship was observed between the frequency of chromosomal aberrations and the clinical stage and activity of the disease. In most of the patients with resistance to chemotherapy, abnormal karyotypes were observed. After the cytogenetic examination, the survival was significantly shorter for patients in whom an abnormal karyotype was identified (median survival 19 months) than for patients in whom only normal metaphases were observed (median survival 42 months).