Red blood cell distribution width (RDW) is a parameter reported in complete blood cell count tests, and has been reported as an inflammatory biomarker. Multiple myeloma (MM) is known to be associated with inflammatory microenvironments. However, the importance of RDW has been seldom studied in MM. For this study, 146 symptomatic myeloma patients with available RDW at diagnosis were retrospectively reviewed, and their characteristics were compared between two groups, those with high (>14.5%) and normal (≤ 14.5%) RDW. RDW was correlated to hemoglobin, MM stage, β 2-microglobulin, M-protein, bone marrow plasma cells, and cellularity (P < 0.001). During induction, overall response rates of the two groups were similar (P = 0.195); however, complete response rate was higher in the normal-RDW group than it was in the high-RDW group (P = 0.005). With a median follow-up of 47 months, the normal-RDW group showed better progression-free survival (PFS) (24.2 versus 17.0 months, P = 0.029) compared to the high-RDW group. Overall survival was not different according to the RDW level (P = 0.236). In multivariate analysis, elevated RDW at diagnosis was a poor prognostic factor for PFS (HR 3.21, 95% CI 1.24-8.32) after adjustment with other myeloma-related prognostic factors. RDW would be a simple and immediately available biomarker of symptomatic MM, reflecting the systemic inflammation.