Magnesium is one of the most abundant cations in the body and is involved in many cell functions. Serum magnesium concentration is maintained within a narrow range by the kidney and digestive tract. It has been reported that a lower serum magnesium level is a significant predictor for mortality in hemodialysis patients. Body composition and inflammation are important predictors of mortality in peritoneal dialysis (PD) patients. The objective of the present study was to examine the relationship of serum magnesium with body composition and inflammation in PD patients. Our study enrolled 62 PD patients treated at the Long Island College Hospital between November 2000 and July 2008. Demographic, clinical, and biochemical data were recorded. Body composition parameters were determined by bioelectrical impedance analysis (BIA). High sensitivity C-reactive protein (hs-CRP), a marker of inflammation was measured by the immunoturbidimetric method. In these patients (mean age: 55 years; 63% African American; 55% women; 25% with diabetes), the mean (+/- standard deviation) serum magnesium and hs-CRP were 1.597 +/- 0.28 mEq/L and 13.70 +/- 21 mg/L respectively. Serum magnesium was directly correlated with serum markers of nutrition: albumin (r = 0.42, p = 0.001), creatinine (r = 0.43, p = 0.0001), and total protein (r = 0.44, p < 0.0001). Serum magnesium was also directly correlated with phase angle, a BIA parameter and marker of cellular health (correlation coefficient: r = 0.35; p = 0.006), and inversely correlated with the extracellular mass/body cell mass ratio (r = -0.34, p = 0.008), a highly sensitive marker of malnutrition. We observed an inverse correlation between serum magnesium and hs-CRP (r = -0.37, p = 0.02) in PD patients. In conclusion, lower serum magnesium is associated with poorer nutrition status, deteriorating cellular health, and increased inflammation, which may contribute to the increased risk of mortality in PD patients.