The distinction between upper versus lower urinary tract infection in patients with candiduria is a commonly encountered and therapeutically important diagnostic dilemma. Candida casts have been reported in the urine of several individual case reports of human renal candidiasis. The specificity of Candida casts would identify unequivocally a patient with upper urinary tract disease. Little is known, however, about the sensitivity and the formation of Candida casts. We therefore studied the diagnostic yield, methods for detection and pathogenesis of Candida cast formation in serially collected urine specimens from immunologically intact and granulocytopenic rabbit models of haematogenous disseminated candidiasis. Refractile blastoconidia and pseudohyphae of Candida encased in the granular matrix were seen on wet mounts while Candida stained a brilliant red in the fuschia pink tubular matrix on periodic acid Schiff (PAS) stained cytopathology filters. Among 24 rabbits with disseminated candidiasis, 11 (46%) had Candida casts detectable by wet mount and PAS-stained urine filters in comparison to none of 10 non-infected immunologically normal controls (P = 0.014). Fifteen (70%) of 21 episodes of Candida casts were detected within the first 3 days of infection, indicating possible utility in the early diagnosis of renal candidiasis. No Candida casts were detected in the urine of granulocytopenic rabbits, possibly due to the rapid destruction of tubules and abrogation of cast formation. This absence of detectable Candida in eight infected granulocytopenic rabbits differed significantly from that of 24 non-granulocytopenic infected rabbits, in which Candida casts were detected in 11 (46%) (P = 0.029). Candida cast formation occurred predominantly in the cortex. Histopathological examination demonstrated invasion of Candida into the glomerular tufts and peritubular capillaries, followed by development of Candida casts in the proximal and distal tubules, respectively. Detection of renal Candida casts may be a useful diagnostic marker in distinguishing upper versus lower urinary tract candidiasis.