Nafcillin is a semisynthetic penicillin extensively used for treatment of infections caused by penicillinase-producing staphylococci. It has been known to cause adverse reactions such as hypersensitivity reactions, rash, and phlebitis. However, there are only a few reports that describe nafcillin as a possible cause of hypokalemia (JAMA. 1979;242:544, J Pediatr. 1980; 97:841-842). It is thought to be largely due to nafcillin acting as an impermeant, nonreabsorbable anion, increasing potassium excretion. This report describes a patient treated with nafcillin for methicillin-susceptible Staphylococcus aureus psoas abscess who developed hypokalemia with apparent renal potassium wasting.