Methicillin-resistant Staphylococcus aureus (MRSA) bacteriuria was detected in 11 of 41 patients colonized or infected with MRSA. The patients with bacteriuria generally were older than 40 years of age, five were diabetic, seven had prior indwelling urethral catheters, two had undergone other urologic manipulations, and only one was clinically symptomatic. Eight patients received variable combinations of antibiotic therapy prior to the diagnosis of MRSA bacteriuria, and seven were still on antibiotic therapy at the time the bacteriuria was detected. Bacteriuria lasted four days to 14 weeks, and was eradicated promptly with cephalosporin therapy in five of six patients. Bacteriuria in the untreated patients cleared spontaneously in one month. A single MRSA serotype (83A) predominated. The MRSA isolates were resistant in vitro to most antibiotics except vancomycin. Resistant colonies were observed within cephalothin disc zones of inhibition at 30C (resistance was confirmed by microtube-dilution sensitivity testing). MRSA disc sensitivity testing for cephalothin may be unreliable when performed at 35C. (Am J Med Sci 281(2):101–109.)