Introduction: Gemcitabine, a chemotherapeutic agent, has been shown to be active against transitional cell cancer of the bladder. The aim of the study was to determine the pharmacokinetic profile of gemcitabine, administered intravesically in patients with carcinoma in situ(CIS). Material and Methods: Nine patients with CIS refractory to intravesical bacillus Calmette-Guérin (BCG) therapy were enrolled. Gemcitabine was given in 50 ml 0.9% NaCl by catheterization and held in the bladder for 1 h, once weekly for 6 consecutive weeks. The pharmacokinetics for gemcitabine metabolites were performed in plasma and serum. Dose levels were: 1,000, 1,250, and 1,500 mg. Clinical evaluation was repeated 4 weeks after therapy and thereafter every 6 months. Results: Grade-1 neutropenia was observed only in 1 patient. Grade-1 urinary frequency and hematuria were observed in 1 and 3 patients, respectively. No grade 2–4 toxicity or clinically relevant myelosuppression were observed. Gemcitabine was detectable in serum, but with an irrelevant pharmacological effect, in only 1 patient treated with 1,500 mg of gemcitabine. With regard to activity, after 6 instillations of this drug, 4 complete responses were observed. Conclusion: Intravesical gemcitabine is well tolerated and safe. No systemic absorption with a clinical or pharmacological effect was detected and only slightly irritative bladder symptoms were observed. These results warrant further investigation in phase-II trials.