The aim of this study was to evaluate the efficacy and toxicity of 5-fluorouracil (5-FU) and l-leucovorin (l-LV) given at the same dose intensity and administered monthly (given weekly for 3 weeks followed by a week of rest; arm A) or every 2 months (given weekly for 6 weeks followed by 2 weeks rest; arm B) to patients with advanced colorectal carcinoma. The dose of 5-FU was 500 mg/body or 750 mg/body, with an average dose of 432.8 mg/m2. A total of 7 institutions participated in this multi-center study and were randomly divided into 2 groups of arms A and B. Thirty-three patients were entered into arm A and 21 into B. The overall response rate was significantly (p = 0.007) greater in arm B (23.5%) than in arm A (0%). The most frequently observed toxicity was diarrhea, which was observed in 6.5% of arm A and in 33.3% of arm B, marking a significant difference (p = 0.034). These data suggest that a monthly 5-FU/l-LV regimen might be less toxic than a 2-months regimen and less effective at the dose given as above. Further study is needed to evaluate the efficacy of a monthly regimen.