In the treatment of one male patient with chronic pyelonephritis, complicated with renal stone, the pathological state of the renal inflammatory lesion was determined. The patient had been persistently infected by the same strain of S. marcescens for more than a year. When he was treated by several antimicrobial agents, the urinary bacteriological response was well correlated to the MICs of each agent. On the basis of the findings obtained, a new index of local antimicrobial activity was proposed. Analysis of such items as strains appearing after treatment, interval of relapse and the identification of the strains relapsed, were suggestive of the renal inflammatory, and pathological conditions. The clinical response also correlated well with the index. The lesion was considered to be mainly localized in the right lower calyx where a tiny stone existed. This disease is considered curable with effective chemotherapy after withdrawal of the stone. This index should be useful for evaluation of the effectiveness of antimicrobial agents.