A phosphate depletion syndrome developed in a steroid-dependent asthmatic patient. Initially, the clinical picture was confused with steroid-associated myopathy rather than the phosphate depletion syndrome which has similar symptoms. The classic biochemical findings led to the correct diagnosis. Cessation of phosphate-binding antacids and phosphorus repletion rapidly corrected the biochemical findings and led to the patient's clinical improvement. Platelet phosphate metabolism was evaluated; it was found to correlate with the phosphorus-depleted state and clinical recovery with phosphate repletion. Attention is drawn to the clinical entity of phosphate depletion which may mimic steroid-induced side effects, both of which may occur in patients receiving steroids and antacids.