Eighty-six eligible patients with non-small cell lung cancer were treated on a Phase II, CALGB study with high-dose, 24-h continuous intravenous 5-fluorouracil every 2 weeks. Objective responses were seen in 7 (8%) patients with I (1 %) complete response and 6 (7%) partial responses. The median survival for these patients without prior chemotherapy was 3.8 months. Gastrointestinal and hematologic toxicity were acceptable for most patients. However, two patients experienced acute clinical deterioration characterized by worsening central nervous system and hemodynamic function beginning near the completion of chemotherapy treatment and resulting in death. Because of its potential for severe, unpredictable neurologic and cardiac toxicity, we do not recommend this dose and schedule of 5-FU for future trials.