The growth modulation index (GMI) is the ratio of time to progression with the nth line (TTPn) of therapy to the TTPn-1 with the n-1th line. GMI > 1.33 is considered as a sign of activity in phase II trials. This retrospective analysis evaluated the concordance between the GMI and the efficacy outcomes in 279 patients with advanced soft tissue sarcoma (ASTS) treated with trabectedin 1.5 mg/m superset of (24-h infusion every 3 weeks) in four phase II trials. One hundred and forty-two (51%) patients received one prior line and 137 >= 2 lines. The median TTPn was 2.8 months (range 0.2-26.8), whereas the median TTPn-1 was 4.0 months (0.3-79.5). The median GMI was 0.6 (0.0-14.4). Overall, 177 patients (63%) had a GMI < 1; 21 (8%) a GMI equal to 1-1.33 and 81 (29%) a GMI > 1.33, which correlated with the median overall survival in those patients (9.1, 13.9 and 23.8 months, respectively, P = 0.0005). A high concordance rate between the GMI and response ra A high GMI was associated with favorable efficacy outcomes in patients treated with trabectedin. Further research is needed to assess GMI as an indicator in this setting.