Oxaliplatin (L-OHP) has been established as a key drug for advanced colorectal cancer, and combination therapy with 5-FU/Leucovorin (LV)(FOLFOX regimen) is commonly used in Europe and the US. A phase I study of modified (m) FOLFOX 6 therapy was conducted to determine the recommended dose (RD) of 5-FU infused for 46 hours. Inclusion criteria were unresectable advanced colorectal cancer,measurable lesions, performance status (PS; ECOG) 0-2, age 20-75 years, and adequate organ functions. L-OHP and l-LV was administered over 2 hours and followed by bolus injection and continuous infusion of 5-FU for 46 hours every 2 weeks. Two cycles of mFOLFOX 6 therapy were performed. Doses of L-OHP, l-LV, and bolus injection of 5-FU were fixed at 85 mg/m(2), 200 mg/m(2), and 400 mg/m(2), respectively. The dose of continuous infused 5-FU was escalated from 1,600 mg/m(2), (level 1), 2,000 mg/m(2), (level 2), 2,400 mg/m(2), (level 3), and 2,800 mg/m(2), (level 4). RD was determined in a dose escalation manner, and safety was evaluated according to NCI-CTC Ver 2.0. A total of 13 patients were enrolled. Male/female=7/6, PS 0/1/2=2/4/7, mean age 64 years (range 55-75). Thrombocytopenia was not observed, and grade 2 of neutropenia and peripheral neuropathy was observed in 4 and 6 out of 13 patients. No dose-limiting toxicity (DLT) was observed at level 1 (n=3), 2 (n=4), and 3 (n=4), but at level 4 (n=2), 2 patients experienced DLT; grade 3 easy fatigue and anorexia required treatment delay over 7 days. Level 3 was therefore determined as RD. A phase II study is ongoing to evaluate the efficacy of mFOLFOX 6 therapy.