The pathogenesis of Candida urinary tract infection (UTI) has been investigated clinically and experimentally with special reference to ascending pyelonephritis in rats. Among the Candida species, Candida albicans was most frequently isolated from clinical specimens including urine in two medical centers, one in Japan and the other in the United States. The isolates of C. albicans serotype B showed a significantly lower susceptibility to 5-fluorocytosine compared to those of serotype A (p less than 0.01). The distribution pattern of the serum antibody titers against C. albicans in 20 candiduria patients (C. albicans 19 and Candida tropicalis 1) was similar to that in 23 bacterial complicated UTI patients. All patients with candiduria had underlying diseases of the urinary tract, such as neurogenic bladder, bladder cancer or benign prostatic hyperplasia: indwelling urinary catheters were present in 15 patients and all had received antimicrobial agents before the study. Ascending Candida pyelonephritis has been investigated in female rats which were transurethrally inoculated into the bladder with C. albicans ATCC 10259 strain. The incidence of Candida pyelonephritis was approximately 80% in rats treated with cyclophosphamide and more than 70% in rats with partial ureteral obstruction. There was a significant relationship between renal and urinary Candida cell populations (p less than 0.01). Furthermore, a significant relationship was revealed between renal Candida cell populations and histological grades of pyelonephritis (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)