Ms. A, a 52-year-old woman with a history of three depressive episodes, was admitted for hyperenergetic, loud, and unfocused behavior. She was also elated and delusional. Two days earlier, she had been prescribed a regimen of clarithromycin, 500 mg b.i.d., and prednisone, 60 mg q.i.d., followed by a taper, for severe sinusitis. Results of a neurologic examination were within normal limits; laboratory data were also within normal range. The results of a drug-of-abuse screen were negative; Ms. A’s blood alcohol concentration was zero. Her clarithromycin was discontinued; the prednisone taper continued. Within 1 week, Ms. A responded to haloperidol and lithium treatment. She was tentatively diagnosed as having steroid-induced psychosis. Lithium was discontinued after 4 months because of the appearance of hypothyroidism, probably induced by lithium. Two years later, again 2 days after starting treatment with clarithromycin for an acute episode of sinusitis, Ms. A became agitated and acutely delusional. She believed she was "Jesus Christ," a singer and a superstar. She would scream intermittently. She had an expansive mood, flight of ideas, an intense affect, and looseness of associations, but no hallucinations. Her physical examination and laboratory results were unremarkable. Clarithromycin was discontinued, and Ms. A was started on a regimen of haloperidol and lithium. An unequivocal improvement was noted the next day with mildly pressured speech and an expansive mood, but a denial of delusions. Ms. A was discharged 6 days after her admission.