We studied the serum phosphorus (P) level of 110 patients with multiple myeloma (MM) (age range 42-83 years, median 62 years) and evaluated the relationship between that and other prognostic factors. Serum P level significantly correlated with the prognostic factors that are relevant to renal dysfunction: serum creatinine (P<0.00000001), serum beta2-microglobulin (P=0.00000088), serum uric acid (P=0.0000014), and corrected serum calcium (cCa P=0.000067). Although it also correlated with the percentage of plasma cells in bone marrow nucleated cells (BMPC%) and the hemoglobin (Hb) and leukocyte counts, the significance was less than for the other four prognostic factors. Serum creatinine, BMPC%, leukocyte count, serum uric acid, bone lesions, beta2-microglobulin, and serum cCa were all significantly higher and Hb significantly was lower in the MM patients with hyperphosphatemia (serum P>3.8 mg/dl). The survival time was significantly shorter in these patients (P=0.000087). Multivariate analysis (Cox's proportional hazards regression model) showed that the serum P level is a significant negative prognostic factor in MM patients.