Abnormalities in serum phosphate levels are more prevalent in certain subsets of Emergency Department patients than in the general population. Patients with diabetic ketoacidosis, chronic obstructive pulmonary disease, alcoholism, malignancy, and renal failure are at increased risk. Multiple factors, including nutritional intake, medications, renal or intestinal excretion, and cellular redistribution, are potential etiologies. The clinical manifestations of mild hypophosphatemia or hyperphosphatemia are typically minor and nonspecific (myalgias, weakness, anorexia). When the imbalance is severe, critical complications may occur (tetany, seizures, coma, rhabdomyolysis, respiratory failure, ventricular tachycardia). Mild asymptomatic hypophosphatemia can be treated with oral phosphate supplementation (15 mg/kg daily) on an outpatient basis. Patients with severe or symptomatic hypophosphatemia should be treated with IV phosphate therapy (0.08-0.16 mg/kg over 6 h) and admitted for monitoring and subsequent serum electrolyte testing. Mild asymptomatic hyperphosphatemia is commonly managed in renal failure by limiting dietary intake and reducing absorption with phosphate-binding salts. Hemodialysis may be required for severe hyperphosphatemia with symptomatic hypocalcemia.