Chromosome abnormalities were found in 70 bone marrow specimens of 38 multiple myeloma (MM) patients. Structural abnormalities were insignificant except for marker chromosomes in 11 patients. Numerical abnormalities ranged from hypodiploidy to hyperdiploidy, and clones were recognized in 7 patients. The diversity of cytogenetic findings without any specificity for MM or its differing product places this hematologic neoplasm into the group of solid malignant tumors without specific chromosome changes. The chromosome changes varied during treatment. Decrease in the proportion of abnormal karyotypes, with increase in normal or hypodiploid and tetraploid metaphases, occurred in remissions. Increase in abnormal metaphases (especially hyperdiploidy with decrease in diploidy) and appearance of marker chromosomes and clones were often associated with relapse. Comparison of initial studies with subsequent chromosome studies was valuable in the assessment of the patients' condition.