Background: Phase I dose escalation to a maximum planned dose (MPD) o determine dose-limiting toxicities (DLTs), maximum tolerated dose (MTD), recommended-phase-II-dose (RP2D) and preliminary efficacy of sorafenib and FOLFIRI (irinotecan reduced-dose) in metastatic colorectal cancer (mCRC) patients. Methods: Starting doses: irinotecan 80 mg/m² iv d1, sorafenib 400 mg po twice daily (bid, continuous), starting day 2. Escalations based on toxicity observed at the previous dose level (DL) up to: irinotecan 100 mg/m² and sorafenib 800 mg bid. DLT was evaluated within the 1^st study cycle (1 cycle = 2 FOLFIRI treatments). Results: 5 cohorts were concluded. All 16 ECOG PS 0-1 patients (9/7 men/women; 2/14 rectal/colon) with median age of 64, discontinued study: 10 (62%) disease progression, 4 (25%) toxicity, 1 curative surgery, 1 comorbidities. The dose levels of irinotecan (mg/m², day1) and sorafenib (mg/day, bid, day 2-28) studied were DL1-80/400, DL2-80/600, DL3-90/600, DL4-100/600 and DL5-100/800, repeated every 4 weeks, 3 patients/DL. No DLT was observed. The MTD was not reached at the MPD (DL5). The most common ≥Gr2 treatment related adverse events (AEs) were: neutropenia 81%, HFS 69%, leucopenia 50%, fatigue 38%, anemia 31%, constipation 31%, nausea/vomiting 31%, mucositis 31%, diarrhea 25%, hypophosphatemia 25%. The most severe treatment related AEs were: Gr4: neutropenia 2 (12.5%); pulmonary embolism 1 (6%); Gr3: HFS 9 (56%), neutropenia 3 (19%), leucopenia 3 (19%), hypophosphatemia 3 (19%). Objective response rate was 56% (9 of 16 patients, 95%CI; 33-77%). Response duration was 13 months (95%CI; 5-17). Median progression-free survival and overall survival were 11 months (95%CI; 6-17) and 25 months (95%CI; 15-34), respectively. Conclusions: Combination therapy with S and modified FOLFIRI in these patients is well tolerated and demonstrates clinical efficacy at the MPD. The MTD was not reached at the MPD. Future research of this combination is warranted. Supported by Bayer Healthcare Pharmaceuticals. Clinical trial information: NCT00780169.