The major purposes of this study were to determine the maximally tolerated dose (MTD), dose-limiting toxicity (DLT), toxicity profile, and antitumor activity of gemcitabine (GEM) (Gemzar) and 5-fluorouracil (5-FU) combination therapy when administered to patients with advanced solid tumors. GEM was administered intravenously over 30 minutes on days 1, 8, and 15, and 5-FU was administered as a continuous intravenous infusion from day 1 through day 15 of each 28-day treatment course. Seventeen patients (13 men and 4 women, median age 57, all previously treated with chemotherapy) were treated with 68 courses at 3 dose levels: 800/200, 1,000/200, and 1,000/300 [GEM (mg/m2/week)/ 5-FU (mg/m2/day)]. Two further patients were not fully evaluable for toxicity; one died from a probable pulmonary embolism, and one refused further treatment after developing grade II mucositis and dermatitis after her day 1 to 7 treatment. At the third dose level, 2 of 4 patients developed grade III mucositis; one also developed grade IV neutropenia with fever and grade III thrombocytopenia. Patient accrual then resumed at the second dose level. At this level, 10 patients were treated, with two developing grade III mucositis. One of these patients also developed grade IV dermatitis. No other patient developed grade III or IV side effects. Prophylactic dexamethasone was initiated after 4 of the first 7 patients (including 1 of the not fully evaluable patients) developed dermatitis-grade IV in 1 patient and grade II in the remaining 3 patients. After the steroids were initiated, 4 of the last 11 patients treated developed dermatitis, but grade 1 in all cases. One patient with metastatic gastric cancer achieved a near-complete response of his gastric mass and adrenal metastasis. Minor responses were achieved in a patient with colon carcinoma and a patient with an ethmoid sinus adenoid cystic carcinoma. The MTD and recommended dose for phase II clinical trials of GEM and 5-FU on the above schedule is 1,000 mg/m2 and 200 mg/m2 respectively, with mucositis as the DLT.