Flavopiridol is a semi-synthetic flavone analog of the alkaloid, rohitukine, a compound from an Indian tree, Dysoxylum binectariferum. It has been shown to inhibit cyclin-dependent kinases (CDKs), causing cell cycle arrest and growth inhibition. Flavopiridol is reported to have cytotoxic activity against a wide range of cancer cell lines and has demonstrated its efficacy in several clinical trials. Flavopiridol seems a well-suited potential new agent for the treatment of bladder cancer. We, therefore, evaluated whether flavopiridol inhibits growth and induces apoptosis in bladder cancer cells and additionally examined the toxicity and efficacy of this drug in vivo in a rat bladder cancer model. The in vitro experiments showed an IC20 of 50-100 nM in all cell lines tested. However, there was a difference in the response with regard to the grading of the tumor cells at higher doses. The IC50 was found to be 150-350 nM in the well-differentiated RT4 and RTI12 cell lines after treatment with flavopiridol, in comparison to a IC50 of 1000 nMfor the poorly-differentiated cell lines T24 and SUP. After exposure to flavopiridol, all tumor cell lines underwent significant apoptosis in comparison to untreated cells, beginning at a dose of 50 nM flavopiridol. At high concentrations (500 nM) of flavopiridol, 80-90% of all cells showed severe apoptotic alterations. The treatment of rat urinary bladder cancer with flavopiridol demonstrated the best efficacy with an intermittent treatment of 0.1 mg/kg, 3 times weekly over a total of 3 weeks, resulting in 7/12 animals tumor-free and a trend for the remaining tumors to have lower stage and grade. There seems to be a small advantage in intermittent versus daily application of flavopiridol. In summary, our results indicated that flavopiridol could be a useful therapeutic agent for bladder cancer, inhibiting tumor growth, malignant progression and inducing apoptosis.