Purpose: This double-blind, phase III study aimed to demonstrate that sunitinib plus FOLFIRI (fluorouracil, leucovorin, and irinotecan) was superior to placebo plus FOLFIRI in previously untreated metastatic colorectal cancer (mCRC). Patients and Methods: Patients were randomly assigned to receive FOLFIRI and either sunitinib (37.5 mg per day) or placebo (4 weeks on treatment, followed by 2 weeks off [schedule 4/2]) until disease progression. The primary end point was progression-free survival (PFS). Secondary end points included overall survival, safety, and patient-reported outcomes. The correlation between genotype and clinical outcomes was also analyzed. Results: In all, 768 patients were randomly assigned to sunitinib plus FOLFIRI (n = 386) or placebo plus FOLFIRI (n = 382). Following a second prespecified interim analysis, the study was stopped because of potential futility of sunitinib plus FOLFIRI. Final results are reported. The PFS hazard ratio was 1.095 (95% CI, 0.892 to 1.344; one-sided stratified log-rank P = .807), indicating a lack of superiority for sunitinib plus FOLFIRI. Median PFS for the sunitinib arm was 7.8 months (95% CI, 7.1 to 8.4 months) versus 8.4 months (95% CI, 7.6 to 9.2 months) for the placebo arm. Sunitinib plus FOLFIRI was associated with more grade ≥ 3 adverse events and laboratory abnormalities than placebo (especially diarrhea, stomatitis/oral syndromes, fatigue, hand-foot syndrome, neutropenia, thrombocytopenia, anemia, and febrile neutropenia). More deaths as a result of toxicity (12 v four) and significantly more dose delays, dose reductions, and treatment discontinuations occurred in the sunitinib arm. Conclusion: Sunitinib 37.5 mg per day (schedule 4/2) plus FOLFIRI is not superior to FOLFIRI alone and has a poorer safety profile. This combination regimen is not recommended for previously untreated mCRC.