Loss of heterozygosity (LOH) and alterations in microsatellite DNA markers have been reported in bladder-cancer tumors. We have studied, in a blinded fashion, using PCRbased microsatellite analysis, genetic alterations of cells exfoliated in urine of 59 Caucasian patients and control patients; 31 with initially confirmed bladder transitional-cell carcinoma (TCC), 17 with signs and symptoms suggestive of bladder cancer, 6 control patients who underwent renal transplantation, and 5 control patients with urolithiasis. Microsatellite analysis of cells exfoliated in the urine allowed the diagnosis of 83% (10/12) of patients with bladder TCC recurrence confirmed by cystoscopy, while 100% of patients followed up for transitional-cell carcinoma of the bladder for up to 12 months without evidence of tumor recurrence upon routine cystoscopy showed no microsatellite alterations. None of the patients without neoplasia (negative controls) had any microsatellite alterations, whereas all patients who underwent renal transplantation had additional new alleles corresponding to contamination with donor's renal and urothelial cells (positive controls). No control patients had any evidence of transitional-cell carcinoma by cystoscopy. Our results provide objective evidence that non-invasive molecular detection of bladder TCC by microsatellite analysis is reproducible with a sensitivity of 83% and a specificity of 100% in Caucasian patients. This non-invasive procedure represents a potential clinical tool for the detection and the screening of bladder TCC.