Antibiotic treatment of urinary tract infections is less than completely successful. Factors involved in the outcome are complex and include host, bacterial and drug factors. Eradication of the bacterial strain from the urine after the discontinuation of drug administration is a critical index in the clinical evaluation. Categorical reports of in vitro bacterial susceptibility correlate poorly with clinical results. Achievement of a bacterial inhibitory concentration of drug in the urine is essential for cure and probably more important than the plasma concentration. The conditions associated with cure and failure in 368 episodes of infection were examined. Distribution of the patients into groups designated "anatomical normal", "systemic nephropathy" and "uronephropathy" showed each of these host factors to have a significant primary effect. In the latter two, but not in the former, the drug susceptibility of the infecting bacterial strain and whether or not it was E. coli or another species of gram-negative bacillus were additional significant factors. When antibiotic treatment was assessed in host comparable episodes a significant rank order of the effectiveness of drugs could be made according to their class action. Bactericidal inhibitors of protein synthesis were best; drugs used primarily as urinary antiseptics were the least effective.