Purpose To determine whether adding cetuximab to irinotecan prolongs survival in patients with metastatic colorectal cancer (mCRC) previously treated with fluoropyrimidine and oxaliplatin. Patients and Methods This multicenter, open-label, phase III study randomly assigned 1,298 patients with epidermal growth factor receptor–expressing mCRC who had experienced first-line fluoropyrimidine and oxaliplatin treatment failure to cetuximab (400 mg/m² day 1 followed by 250 mg/m² weekly) plus irinotecan (350 mg/m² every 3 weeks) or irinotecan alone. Primary end point was overall survival (OS); secondary end points included progression-free survival (PFS), response rate (RR), and quality of life (QOL). Results Median OS was comparable between treatments: 10.7 months (95% CI, 9.6 to 11.3) with cetuximab/irinotecan and 10.0 months (95% CI, 9.1 to 11.3) with irinotecan alone (hazard ratio [HR], 0.975; 95% CI, 0.854 to 1.114; P = .71). This lack of difference may have been due to post-trial therapy: 46.9% of patients assigned to irinotecan eventually received cetuximab (87.2% of those who did, received it with irinotecan). Cetuximab added to irinotecan significantly improved PFS (median, 4.0 v 2.6 months; HR, 0.692; 95% CI, 0.617 to 0.776; P ≤ .0001) and RR (16.4% v 4.2%; P < .0001), and resulted in significantly better scores in the QOL analysis of global health status (P = .047). Cetuximab did not exacerbate toxicity, except for acneform rash, diarrhea, hypomagnesemia, and associated electrolyte imbalances. Neutropenia was the most common severe toxicity across treatment arms. Conclusion Cetuximab and irinotecan improved PFS and RR, and resulted in better QOL versus irinotecan alone. OS was similar between study groups, possibly influenced by the large number of patients in the irinotecan arm who received cetuximab and irinotecan poststudy.