Hypomagnesemia is a common but frequently overlooked electrolyte disorder that occurs as part of a complex metabolic profile. It often is associated with a spectrum of nonspecific symptoms secondary to other electrolyte deficiencies. These symptoms then are associated with the primary illness, thereby masking the presence of this disorder. In most clinical situations, magnesium deficiency is transitory and responds well to short-term supplementation. Certain populations of patients with cancer may have an increased risk of developing severe hypomagnesemia requiring continued supplementation; patient populations receiving aminoglycoside antibiotics or cisplatin therapy are considered to be high-risk groups. This paper describes the causes, signs, and symptoms of chronic hypomagnesemia; patient populations at risk of developing this disorder; and a unique treatment approach using a subcutaneous pump infusion system. A case study illustrates the complexity of clinical and nursing management of this disorder.