53 women with acute, uncomplicated urinary tract infection proven clinically and bacteriologically were treated, after double-blind randomisation, with a single dose of norfloxacin (1200 mg) or a single dose of cotrimoxazole (480/2400 mg). Follow-up one week after treatment showed a comparable success rate (85%) for both antibiotics. Only few minor side effects were recorded in both patient groups. Most failures were due to gram positive bacteria (coagulase-negative staphylococci). These results suggest that single dose therapy for urinary tract infections due to gram positive germs should be reconsidered.