The 100,000 colony count has been used without careful evaluation for the diagnosis of renal candidiasis. Therefore, a prospective study was done on 1,004 urine samples from patients without signs of candidiasis to determine the incidence of candiduria and Candida colony counts. These were compared with colony counts from histologically proved cases of renal candidiasis. The incidence of candiduria varied from a low of 4% in men to a high of 39% in girls during their second week of antibiotic therapy. The overall mean colony count was 1,292 +/- 1,500. The mean colony count in clean-catch urine specimens from six proved cases of renal candidiasis was 23,750 +/- 12,311. The difference in colony counts from proved and unproved cases is statistically significant. Counts greater than 10,000 Candida organisms per milliliter require further investigation.