Pemetrexed (Alimta) shows single-agent activity in advanced colorectal cancer. In two phase II studies in which patients received pemetrexed at 600 mg/m2 or 500 mg/m2 as first-line treatment for metastatic disease, objective response rates were 15.4% and 17.2%. These trials were conducted prior to supplementation with folic acid and vitamin B12, which markedly decreased the frequency of hematologic toxicities of pemetrexed; routine supplementation is now included in all clinical trials of the agent. The marked improvement in toxicity and tolerance with vitamin supplementation suggests the need to reexamine optimal dosing in pemetrexed combination schedules. In a National Surgical Adjuvant Breast and Bowel Project phase II trial in 54 patients with previously untreated advanced colorectal cancer, pemetrexed at 500 mg/m2 plus oxaliplatin (Eloxatin) at 120 mg/m2 every 21 days with folic acid/vitamin B12 supplementation resulted in an objective response of 23%. Three additionalpatients (5.6%) had unconfirmed partial response (partial response at one visit), and 27 patients (50%) had stable disease. Median progression-free survival was 5.3 months and median duration of response was 5.7 months; median overall survival was approximately 11.05 months. Grade 3/4 neutropenia was observed in only 17% of patients and treatment was well tolerated. A phase I/II study is under way to identify and assess the optimal combination of pemetrexed/irinotecan in second-line treatment of advanced colorectal cancer. Planned studies include a phase I study examining the combination of pemetrexed and oxaliplatin given every 2 weeks as first-line treatment, and a phase I/II trial to identify the optimal pemetrexed/ oxaliplatin dose in a 21-day schedule and to compare pemetrexed/ oxaliplatin with FOLFOX4 in first-line treatment of metastatic disease. All of these trials include vitamin supplementation. A phase III trial comparing the every-3-week pemetrexed/oxaliplatin regimen with FOLFOX4 as first-line treatment will be initiated if the outcome of the phase II trial is encouraging.