Magnesium is a divalent cation involved in more than 300 metabolic processes. Magnesium acts as an intracellular regulator of most energy-demanding pathways. Clinical investigation in the human medical field has determined risk factors for hypomagnesemia and its relationship to a number of disease processes. Experimental studies have established the effects of hypomagnesemia in dogs, but little is known of its prevalence, risk factors, or clinical associations in a hospital population of dogs. To study the prevalence, risk factors, and clinical associations of hypomagnesemia in dogs, a retrospective cross-sectional study of dogs admitted to the University of Minnesota Veterinary Teaching Hospital over a 2.5-year period was undertaken. The prevalence of hypomagnesemia in the study population was 6.1% (188 of 3,102 dogs). Using both categorical and continuous variables in a univariate analysis, significant associations with hypomagnesemia were identified and used to construct a multivariate analysis of the relative risk of hypomagnesemia. Results from evaluation of 3,102 dogs indicate that the most significant predictors of hypomagnesemia were albumin (P < or = .0001; odds ratio [OR] = 0.2), potassium (P < or = .0001; OR = 0.5), total CO2 (P < or = .05; OR = 0.9), and blood urea nitrogen concentrations (P < .0001; OR = 0.9), a diagnosis of cardiovascular disease (P < .02; OR = 1.9); and being a Collie (P < .02; OR = 3.9) or German Shepherd Dog (P < .002; OR = 2.2). These results can be used to better understand and predict hypomagnesemia in dogs.