<b><i>Objectives:</i></b> The inflammation-based Glasgow Prognostic Score (GPS) is associated with outcome in a variety of cancers. This study investigated whether a modified GPS (mGPS) could predict survival in patients undergoing multimodality therapy for advanced colorectal cancer (CRC). <b><i>Methods:</i></b> We enrolled 245 patients with advanced CRC who received chemotherapy. The mGPS was recorded prior to first-line chemotherapy and to cytoreductive therapy including secondary surgery and/or radiofrequency ablation. The prognostic significance of the mGPS was analyzed using Kaplan-Meier, univariate, and multivariate analyses. <b><i>Results:</i></b> In patients who received chemotherapy alone (n = 163), the mGPS prior to chemotherapy was an independent prognostic indicator of survival [odds ratio (OR) 1.858; 95% confidence interval (CI) 1.213–2.846; p = 0.0044]. In patients who also underwent cytoreductive therapy (n = 82), the mGPS decreased after chemotherapy in 22 patients (27%) and increased in 5 (6%). In these patients, the mGPS prior to cytoreductive therapy was an independent prognostic indicator of survival (OR 3.412; 95% CI 1.198–9.720; p = 0.0216), but the mGPS prior to chemotherapy was not. <b><i>Conclusions:</i></b> The mGPS is an independent prognostic indicator of survival in patients undergoing multimodality therapy for advanced CRC, if recorded at a relevant time point.