Total serum magnesium concentration ([Mg2+]s) was prospectively determined for 57 cats admitted to the intensive care unit (ICU) of the Cornell University Hospital for Animals. Data were collected and analyzed to determine the following: prevalence and incidence of [Mg2+] abnormalities, medical disorders associated with altered [Mg2+]s, association of altered [Mg2+]s with other electrolyte abnormalities, length of hospitalization for cats with abnormalities of [Mg2+]s versus those with normal [Mg2+]s, and survival of cats with abnormal [Mg2+)s versus those with normal [Mg2+]s. The point prevalence of magnesium abnormalities was 26%, the period prevalence was 46%, and the cumulative incidence was 23%. Hypermagnesemia was associated with abnormalities of serum potassium (P = .04) and phosphate (P = .01) concentrations. Abnormalities of [Mg2+]s were not associated with abnormal serum concentrations of Na+, Ca2+, or Cl-. On admission. hypomagnesemia was detected in cats with gastrointestinal, endocrine, and other disorders; hypermagnesemia was detected only in cats with renal disease, obstructive uropathy, or neoplastic disease. The median hospital stay for cats that developed abnormal [Mg2+]s after admission was longer than for cats that remained normomagnesemic (5 versus 4 days, respectively; P = .03). Despite the longer hospital stay, the survival of these cats was lower than that of normomagnesemic cats (54 versus 77%; P = .05). When all cats were considered, the survival of cats with abnormal [Mg2+]s also was decreased compared with normomagnesemic cats (62 versus 81%; P = .05). We conclude that abnormalities of [Mg2+]s may affect morbidity and mortality of affected cats.