5-methyltetrahydrofolate (MTHF) is a main serum metabolite of 5-formyltetrahydrofolate (folinic acid, FA), a standard agent for potentiation of the cytotoxic activity of 5-fluorouracil (5-FU). The clinical application of MTHF instead of FA as a precursor of the biologically active metabolite 5,10-methyltetrahydrofolate (mTHF) is based on favorable pharmacologic characteristics of MTHF described so far. In this phase I study 18 patients with advanced solid malignancies were treated with MTHF for 5 days at doses ranging from 100 to 500 mg/m(2)/day in combination with a fixed dose of 500 mg/m2/day 5-FU given as a 4-hour infusion. The treatment was repeated after 21 days. The toxicity observed was mainly gastrointestinal with loss of appetite, nausea and vomiting (up to WHO grade III), and less frequently stomatitis, decline of hemoglobin and hematuria (up to WHO grade II). The frequency and severity of side effects seen were not related to the dose of MTHF. Cumulative toxicity was not observed. The MTD was not reached up to an MTHF dose of 500 mg/m(2)/day. Objective remissions were not seen. The study was terminated on the basis of results showing comparable 5,10-methylenetetrahydrofolate (mTHF) tumor- and tissue levels after administration of MTHF or FA.