Bone marrow flow cytometric analysis is a powerful and rapid tool for evaluating plasma cell myeloma. By using a noncontrolled patient population in various stages of diagnosis and treatment, we compared 6-color (single-tube) and 4-color (multiple-tube) flow cytometric immunophenotyping protocols. Prospective comparison in 52 cases demonstrated improved ability to detect clonal plasma cells or identical diagnoses in 100% of the cases using 6-color, single-tube analysis. In cases in which 6-color flow cytometric analysis improved detection of a clonal population, concurrent biopsy showed less than 5% involvement by plasma cell myeloma, suggesting that 6-color flow cytometry has an advantage in patients with a low disease burden. In addition, the simplification of the procedure resulted in substantial savings in technologist time and reagent costs. Taken together, this study demonstrates that 6-color flow cytometry is an excellent, cost-effective means to assay for clonal plasma cells in a noncontrolled patient population.