Advanced transitional cell carcinoma (TCC) of the urothelial tract is usually fatal despite high response rates to platinum-based chemotherapy regimens. Paclitaxel (Taxol; Bristol-Myers Squibb Company, Princeton, NJ) has demonstrated marked single-agent activity in TCC, and combinations of carboplatin and paclitaxel have been well tolerated in other solid tumors. Methotrexate is also active in TCC. Due to unexpectedly severe myelosuppression and mucositis when methotrexate and paclitaxel were combined, we undertook a phase I trial of paclitaxel, carboplatin, and escalating doses of methotrexate with granulocyte colony-stimulating factor and leucovorin support in advanced TCC to determine the feasibility of this combination. Nineteen previously untreated patients with locally advanced or metastatic TCC were eligible. Median age was 62 years. In sequence, paclitaxel 200 mg/m2 (3-hour infusion), carboplatin dosed to an area under the concentration-time curve of 6 mg/mL x min, and methotrexate 10 mg/m2, increasing in 10-mg/m2 increments, were administered on day 1 every 21 days. Granulocyte colony-stimulating factor 300 microg/d or 480 microg/d (in patients <60 kg or >60 kg, respectively) was administered on days 2 through 11 and leucovorin 15 mg orally every 6 hours for 3 days. At this time, the methotrexate dose has been escalated to 50 mg/m2. There were no dose-limiting toxicities in cycle 1. Sixty-eight cycles have been administered (range, one to eight cycles; median, three cycles). Significant hematologic toxicity including neutropenic sepsis (two episodes) occurred in subsequent cycles, but was infrequent. The major nonhematologic toxicity was neuropathy. Sixteen patients are evaluable for response. One patient has achieved a complete response, seven are partial responders, seven have stable disease, and one progressed on therapy. The overall response rate is 50% (95% confidence interval, 25% to 75%). The combination of paclitaxel, carboplatin, and methotrexate holds promise to be well tolerated and active in advanced TCC.