The aim of this study was to define the maximum tolerated dose (MTD) of bolus mitomycin C (MMC) in combination with 24 h-continuous infusion of 5-fluorouracil (FU) plus folinic acid, and to assess the toxicity and activity in patients with previously treated colorectal and gastric cancer. Escalating doses of MMC starting from 6 mg m(-2) in 2 mg m(-2)-steps to a maximum of 10 mg m(-2) were applied on days 1 and 22, given to fixed doses of 5-FU (2.600 mg m(-2)) as 24 h infusion and folinic acid 500 mg m(-2) prior to 5-FU weekly for 6 weeks