Results from experimental systems suggest that the combination of methotrexate (MTX) and 5-fluorouracil (5-FU) produces synergistic cytotoxic effects that are dependent on the sequence of drug administration. Biochemical studies have demonstrated plausible mechanisms of a synergistic effect when MTX precedes 5-FU. A variety of phase I and phase II studies have employed this sequential combination with promising but inconclusive results. In this study, 79 evaluable patients with squamous cell head and neck cancer were randomized to receive MTX (200 mg/m2 IV) and 5-FU (600 mg/m2 IV) either simultaneously or sequentially with MTX preceding 5-FU by one hour. All patients were ambulatory and patients were stratified as to whether they presented with primary disease or with recurrence after prior radiation and/or surgical therapy. The overall response rate was superior for simultaneous therapy (62%) compared with sequential therapy (38%) (p less than 0.06). Among patients with primary head and neck cancer, the stage of the disease influenced the response rate. Eight of nine stage III patients who received simultaneous therapy responded while none of the four patients who received sequential therapy responded (p less than 0.01). This study demonstrates that one-hour sequential MTX plus 5-FU is not superior to simultaneous treatment in patients with squamous cell head and neck cancer.