Chromosome studies were done on 82 patients with multiple myeloma, 11 with amyloidosis, 2 with multiple myeloma and amyloidosis, and 5 with plasma cell leukemia to investigate their chromosomal abnormalities and to determine the usefulness of cytogenetic studies. A chromosomally abnormal clone was found in 29 patients but was observed most often in those with active disease: in 18% of patients with newly diagnosed multiple myeloma, in 63% with aggressive disease, and in 40% with plasma cell leukemia. Survival among the newly diagnosed patients was significantly shorter (P = .0089) for those in whom an abnormal clone was identified (median survival, six months) than for those in whom only normal metaphases were observed (median survival, greater than 12 months). Among all of the patients, survival from the time of chromosome analysis was shorter for those in whom a chromosomally abnormal clone was found: the median survival was three months for patients with all abnormal metaphases and eight months for patients with normal and abnormal metaphases and has not yet been reached for patients with only normal metaphases. The most common anomalous chromosomes in patients with a plasma cell proliferative disorder were 1, 11, and 14: 11 patients had an abnormality involving chromosome 14q32 and nine patients had an anomalous chromosome 11. The single most common abnormality, a t(11;14)(q13;q32), occurred in three patients. Among the patients who developed preleukemia or acute nonlymphocytic leukemia, the most common anomaly involved chromosome 7. The results suggest that cytogenetic studies are useful for identifying patients who have a poor prognosis and can help distinguish patients with a cytopenia because of preleukemia from those with an aggressive plasma cell proliferative process.