Early erythroid precursor cells and myeloid progenitor cells (CFU-GM) from four patients with 5q-syndrome were cultured in order to study the in vitro growth patterns and to determine the clonal origin of this hematologic disorder. Cultures of CFU-GM exhibited normal colony growth, while erythroid progenitor cells demonstrated a marked decrease or absence of colony growth. Chromosomal studies indicate the 5q- chromosome is present in both hematopoietic progenitor cells, suggesting that the syndrome is an acquired clonal disease arising from a pluripotent hematopoietic stem cell. Follow-up cytogenetic studies reveal a decrease in the number of normal metaphases. This finding is consistent with reports that emphasize the slowly progressive nature of this malignant stem cell defect.