Objective: The aim of our study was to investigate if common toxicities are correlated to objective response rate (ORR) in metastatic colorectal cancer (mCRC) patients treated by irinotecan based regimens. Methods: Univariate and multivariate logistic regression analyses were performed to evaluate correlations between common toxicities and binary ORR in 106 mCRC patients from a prospective cohort treated with irinotecan based regimens. Results: The most frequent severe toxicities (Grade 3/4) were as follows: neutropenia (27.4%), diarrhea (16.9%), leucopenia (12.6%), vomiting (3.2%) and thrombocytopenia (2.1%). Thrombocytosis was observed in 25 (26.3%) patients. ORR was 25.3%. Thrombocytopenia (P = 0.014), line of chemotherapy (P = 0.028) and thrombocytosis (P = 0.033) were correlated with ORR in univariate analysis. In multivariate analysis, thrombocytopenia (odds ratio [OR] = 8.600, 95% confidence interval [CI] = 1.705-43.385, P = 0.009) and first line chemotherapy (OR = 5.155, 95% CI = 1.153-23.256, P = 0.032) positively related to ORR. Conclusion: Thrombocytopenia may be an indicator of ORR in mCRC patients treated by irinotecan plus 5-fluorouracil/capecitabine. Evidence is not strong enough to prove that irinotecan based regimens-induced diarrhea, leucopenia, neutropenia or vomiting is associated with ORR. © 2013 Springer-Verlag Berlin Heidelberg.