Three hundred and forty-five clinical isolates from patients treated with cefpirome were studied using an MIC methodology to develop tentative breakpoints for cefpirome, and to correlate clinical success with in-vitro susceptibility. No relationship between elimination, persistence and MIC of potential pathogens was found. There was, however, a close relationship between clinical cure and improvement and eradication of causative organisms. Based on the data, tentative breakpoints of less than 4 mg/L for sensitive and greater than 16 mg/L for resistance, at a cefpirome dose of 2.0 g bid or 1.0 g bid for urinary tract infection, are suggested.