We present a case of acute psychosis in a 46-year-old woman who had been treated orally with cotrimoxazole because of a severe infection of the urinary tract. She had started to develop psychotic symptoms with bizarre behavior two days before admission. Following discontinuation of antibiotic therapy, including cessation of treatment with cotrimoxazole and the induction of antipsychotic treatment, her mental state resolved to a stable premorbid level within 36 hours.