Real-time PCR technology is highly advantageous for gene studies based on the genetic nature of the transferred material. Urine and blood samples were collected before and after treatment. Treatment of bladder carcinoma patients with plasmid constructs expressing the diphtheria toxin gene was monitored. Detection range from 5x10(6) copies to <or=10 copies was observed with repeatability and reproducibility. No trace amount of the plasmid was detected in the patient's bloodstream, and a continuous clearance in urine, as well, was observed 72 h post-injection. These results show the possibility to design a follow-up method to monitor quantitatively the amount of the plasmid DNA in bladder cancer patients.