Background: Indisetron is a serotonin (5-hydroxytryptamine type 3) receptor antagonist that also antagonizes 5-hydroxytryptamine type 4 receptors. We designed a pilot study in order to explore the optimal dosing period for indisetron during modified FOLFOX6 (mFOLFOX6). Patients and Methods: Forty-two chemotherapy-naive patients with advanced colorectal cancer scheduled to receive mFOLFOX6 were randomly assigned to either a 1- or 3-day indisetron regimen arm. The primary endpoint was complete protection from vomiting. Results: Proportions of patients with complete protection from vomiting were 85.7% [95% confidence interval (CI) 63.7–97.0] with the 3-day regimen and 81.0% (95% CI 58.1–94.6) with the 1-day regimen. Proportions of patients with complete protection from nausea were 47.6% in each arm (95% CI 25.7–70.2). No rescue therapy rates were 66.7% (95% CI 43.0–85.4) versus 57.1% (95% CI 34.0–78.2). No severe adverse events were observed in either arm. Conclusion: Both 1- and 3-day indisetron regimens were feasible for preventing nausea and vomiting induced by mFOLFOX6.