Prior studies have shown the association of low serum magnesium levels with adverse health outcomes in patients undergoing hemodialysis. There is a paucity of such studies in patients undergoing peritoneal dialysis (PD).Cohort study.10,692 patients treated with PD from January 1, 2007, through December 31, 2011, in facilities operated by a single large dialysis organization in the United States.Baseline serum magnesium levels, examined as 5 categories (<1.8, 1.8-<2.0, 2.0-<2.2 [reference], 2.2-<2.4, and ≥2.4mg/dL).Time to first hospitalization and time to death using competing-risks regression models.The distribution of baseline serum magnesium levels in the cohort was <1.8mg/dL, 1,928 (18%); 1.8 to <2.0mg/dL, 2,204 (21%); 2.0 to <2.2mg/dL, 2,765 (26%); 2.2 to <2.4mg/dL, 1,765 (16%); and ≥2.4mg/dL, 2,030 (19%). Of 10,692 patients, 6,465 (60%) were hospitalized at least once and 1,392 (13%) died during follow-up (median, 13; IQR, 7-23 months). Baseline serum magnesium level < 1.8mg/dL was associated with higher risk for hospitalization and all-cause mortality after adjustment for demographic and clinical characteristics (adjusted HRs of 1.23 [95% CI, 1.14-1.33] and 1.21 [95% CI, 1.03-1.42], respectively). The higher risk for hospitalization persisted upon adjustment for laboratory variables, whereas that for all-cause mortality was attenuated to a nonsignificant level. The greatest risk for hospitalization was in patients with low serum albumin levels (<3.5g/dL; P for interaction < 0.001).Possibility of residual confounding by unmeasured variables cannot be excluded.Lower serum magnesium levels may be associated with higher risk for hospitalization in incident PD patients, particularly those with hypoalbuminemia. Additional studies are needed to confirm these findings and investigate whether correction of hypomagnesemia reduces these risks.