In Europe, patients with colorectal carcinoma (CRC) frequently receive mistletoe extracts to improve quality of life and survival. This study was designed to evaluate supportive treatment with mistletoe extract Iscador (ISC) in nonmetastatic CRC patients under routine clinical conditions and to create well-founded hypotheses for future prospective clinical studies. The design of a multicenter, controlled, retrospective, observational cohort study with parallel groups met the Good Epidemiological Practice rules. Anonymous unselected standardized data from eligible patients with surgically treated stage I-III CRC and adjuvant therapy (AT) or conventional aftercare were included. End points were adjuvant therapy-related adverse reactions (AT-ADRs), symptoms, and disease-free survival (DFS). The results were adjusted for confounder effects. Eight hundred four (429 ISC vs 375 control) CRC patients from 26 centers were observed for a median of 58 versus 51 months; the median ISC therapy lasted 52 months. ISC patients showed fewer AT-ADRs (19% vs 48%, p < .001) and fewer persisting symptoms (p < .001). The DFS hazard ratio of 0.60 (p = .013) suggests a survival benefit in ISC patients versus controls. ISC was well tolerated without life-threatening ADRs, drug interactions, or tumor enhancement. These results suggest a beneficial effect of supportive care ISC therapy within AT protocols and long-term ISC treatment in stage I-III CRC patients, particularly improvement in AT-ADRs and symptoms and possible extension of DFS.