Motivated by the observation of preclinical synergy, a Phase I dose escalation study of edatrexate in combination with a 3-h paclitaxel infusion was performed in patients with advanced breast cancer to determine the maximum tolerated dose (MTD) of edatrexate and the toxicities associated with this combination and to report preliminary observations of efficacy with this novel combination. Thirty-six patients were enrolled in this Phase I trial. Thirty-five eligible patients were treated every 21 days in cohorts of at least three patients and were assessable for toxicity. One patient was ineligible due to hyperbilirubinemia. Stepwise dose escalations of edatrexate were administered until grade >3 nonhematological dose-limiting toxicities were reported. The initial dose level of edatrexate was 180 mg/m2; subsequent cohorts were treated with escalating doses of edatrexate (210, 240, 270, 300, 350, and 400 mg/m2). Edatrexate was administered by i.v. infusion over 1 h. Paclitaxel was administered 24 h later at a fixed dose of 175 mg/m2 as a 3-h infusion with standard dexamethasone, diphenhydramine, and cimetidine premedication. The MTD of edatrexate was reached at the 350 mg/m2 level in this study. Grade 3 diarrhea was seen in one patient at the 300 and 400 mg/m2 dose levels, requiring dose reductions. Two patients experienced grade 4 stomatitis at the 400 mg/m2 dose level and also required dose reduction, establishing the MTD as 350 mg/m2. Grade 3 nausea and vomiting were noted in two of three patients at the highest dose level. Of 35 patients, 4 patients reported grade 3 myalgias and 1 patient reported grade 3 neurosensory complaints, which were seen mostly at the 350 and 400 mg/m2 dose levels; however, 1 patient reported grade 3 myalgias at 180 mg/m2. No cumulative neurotoxicity was observed, and no patient experienced an allergic reaction to paclitaxel. In 23 patients with bidimensionally measurable disease, there were four complete (17%) and seven partial responses, with an overall response rate of 48% (95% confidence interval, 27-69%). All of the responses were seen in patients who had not received prior chemotherapy for stage IV disease. The median duration of response was not assessable because many responding patients went on to receive high-dose chemotherapy treatment with stem cell support. The combination of edatrexate and paclitaxel for treatment of metastatic breast cancer is a feasible and safe regimen. The MTD of edatrexate was 350 mg/m2 when combined with a 3-h infusion of paclitaxel (175 mg/m2) given 24 h later. Activity was noted even among patients who had relapsed shortly after receiving methotrexate- and/or doxorubicin-containing adjuvant regimens. Additional studies evaluating the sequences and dosing schema for this combination are warranted to improve the response proportion and define the duration of the response.