Paclitaxel (Taxol; Bristol-Myers Squibb Company, Princeton, NJ) has demonstrated significant antineoplastic activity against different tumor types, notably ovarian and breast carcinoma. Two phase II trials of 24-hour paclitaxel infusions in chemotherapy-naive patients with stage IIIB or IV non-small cell lung cancer (NSCLC) reported response rates of 21% and 24%. Leukopenia was dose limiting: as many as 62.5% of patients experienced grade 4 leukopenia. We investigated the efficacy and toxicity of a 3-hour paclitaxel infusion in a phase II trial in patients with inoperable stage IIIB or IV NSCLC. The 58 patients treated (41 men and 17 women) had a median age of 59 years (age range, 25 to 75) and a performance status of 0 through 2. Most patients (72.4%) had stage IV NSCLC. Paclitaxel 225 mg/m2 was infused over 3 hours every 3 weeks with standard prophylactic premedication. Of 50 patients evaluable for response, 12 (24%) had partial remission, 26 (52%) had no change, and 12 had disease progression (24%). Hematologic toxicities were mild: only one patient (2%) developed grade 3 or 4 neutropenia, while 29% had grade 1 or 2. Grade 1 or 2 polyneuropathy affected 56% of patients while only one (2%) experienced severe polyneuropathy. Similarly, grade 1 or 2 myalgia/arthralgia was observed in 63.2% of patients, but only 14.3% experienced grade 3 or 4. Nausea and vomiting were infrequent, with 14% of patients experiencing grade 1 or 2 and only 2% experiencing grade 3 or 4. Paclitaxel is thus an active single agent in this patient population, with a 3-hour infusion proving comparably effective to a 24-hour infusion and superior in terms of the incidence of hematologic and nonhematologic toxicity. Further phase II studies with paclitaxel combined with other drugs active against NSCLC are indicated, and phase III studies comparing paclitaxel with standard chemotherapy remain to be completed.