Background: FOLFOX or FOLFIRI plus Bev is a first-line treatment for MCRC. Recent studies have confirmed that oral S-1 combined with Iri and Bev is equivalent to FOLFIRI plus Bev. Iri is usually administered as a short-term 90-min. infusion. However, the cytocidal activity is S-phase specific, and carboxylesterases, that convert Iri into SN-38, are less likely to become saturated when Iri is given as a long-term infusion. Therefore, a low dose of Iri given as a long-term infusion is expected to enhance antitumor activity. We conducted a randomized phase II study to compare this regimen with FOLFIRI plus Bev. Methods: The subjects comprised 120 chemotherapy-naïve patients with MCRC. The study group received 24-hr infusion of Iri at a dose of 125 mg/m² on days 1 and 15, combined with oral S-1 80 mg/m² on days 1 to 14. The FOLFIRI group received Iri at a dose of 150 mg/m², 5-FU given at a dose of 400 mg/m² as a bolus injection and at a dose of 2400 mg/m² as a 46 hr-infusion, 200 mg/m² of leucovorin on days 1 and 15. Bev was given at a dose of 5.0 mg/kg on days 1 and 15 in both groups. Treatment was repeated every 4 weeks. The primary endpoint was the 1 y progression-free survival (PFS). Secondary endpoints were PFS, response rates (RR), overall survival (OS), and adverse events (AEs). Results: From October 2013 through December 2017, a total of 61 patients assigned to receive IRIS plus Bev (the A group) and 59 patients assigned to receive FOLFIRI plus Bev (the B group) were included in the analysis. The 1y RFS was 37.3% in the A group and 17.0% in the B group (p = 0.0281). The PFS was 10.2 mon in the A group and 10.0 mon in the B group, and the median OS was 27.0 mon and 28.6 mon, respectively (p = 0.26, p = 0.68). RR was significantly higher in the A group (87.0%) than in the B group (61.7%) (p = 0.005). The main grade 3 or 4 AEs were neutropenia (27.8%) and diarrhea (11.1%) in the A group and neutropenia (23.4%) and leukopenia (6.4%) in the B group. Conclusions: Our results showed that Iri, given biweekly as a 24-hour infusion in combination with oral S-1 and Bev, is a highly effective and well-tolerated regimen for the first-line treatment of MCRC. Clinical trial information: UMIN000014664.