Both cisplatin and leucovorin (LV) interact with fluorouracil (5-FU) by increasing intracellular reduced folate levels and thereby the inhibition of thymidylate synthase. Therefore, the addition of LV to cisplatin and 5-FU (PFL) may increase the activity of that combination in head and neck cancer. We treated 31 patients with locally advanced head and neck cancer with two cycles of neoadjuvant PFL consisting of 100 mg/m2 of cisplatin on day 1 followed by a 5-day continuous infusion of 5-FU at 1,000 mg/m2/day and oral LV at 100 mg administered every 4 hours during the entire duration of chemotherapy infusion. Two patients died during neoadjuvant chemotherapy of sudden death and sepsis, respectively, and were not evaluated for response. Of 29 evaluable patients, nine had a complete response (CR), 17 had a partial response (PR), and three had stable disease. Toxicities consisted of mild to moderate myelosuppression and moderate to severe mucositis, necessitating reduction of 5-FU on cycle 2 to less than or equal to 80% of the intended dose in 22 of 29 patients. Administration of LV by repeated oral dosing resulted in total reduced folate plasma concentrations of 5.3 (+/- 2.9) and 6.7 (+/- 3.4) mumol on days 2 and 4 of the 5-FU infusion. The sum of 1-LV and its metabolite 5-CH3-tetrahydrofolate exceeded the concentration of d-LV, consistent with selective absorption of the biologically active 1-stereoisomer from the gastrointestinal tract.