Background: HR after neoadjuvant ChT for high-risk STS has not been prospectively characterized and the prognostic implications remain unclear. This planned analysis of the ISG-STS-1001 trial, which compared neoadjuvant anthracycline plus ifosfamide (AI) and a histology-tailored (HT) ChT, was aimed at characterizing HR after neoadjuvant ChT and investigating association with the risk of recurrence. Methods: Patients registered in the ISG-STS-1001 study (ID: NCT01710176) were included if they had the per-protocol assessment of HR in surgical specimens. The following morphological post-treatment changes were considered: residual stainable tumor cells, necrosis, hemorrhage, sclerosis, sclerohyalinosis, and fibrohistiocytic reaction with haemosiderin. Pathologists at each study site analyzed and expressed each of these changes as a rate within each tumor. Disease-free survival (DFS), which was the trial primary end-point, was the outcome variable. Results: HR was assessed in 201 of 287 randomized patients (47 had re-excision, 7 did not have surgery, and 32 were not evaluable after wide excision), who were followed up for a median of 89 months (IQR 75-102 months). Presence (>1%) of stainable tumor cells (N=194, 96%) was not associated with DFS (HR=1.47, 95%CI 0.36–5.98, P= 0.591). Conversely, presence (≥1%) of necrosis (N=169, 84%) was associated with shorter DFS (HR=3.11, 95%CI 1.36–7.14, P=0.007) and sclerohyalinosis >20% (N=42, 21%) was associated with a lower risk of recurrence (HR=0.51, 95%CI 0.28–0.94, P=0.031). These findings were confirmed when patients who had preoperative radiotherapy (N=32) were excluded. In patients randomized to AI with tumors evaluable for HR (N=98), sclerohyalinosis >20% (N=24, 24.5%) maintained the association with a lower risk of recurrence (HR=0.24, 95%CI 0.09 – 0.67, P=0.007). Finally, the prognostic value of necrosis and sclerohyalinosis for DFS held also when patients registered in the study (187/361 evaluable for HR) were selected in addition to randomized patients. Conclusions: This is the first study that prospectively evaluated HR after neoadjuvant ChT in high-risk STS. Previous classifications of HR were based on expert consensus and had limited prognostic relevance. In this study residual stainable tumor cells, which is often considered the most relevant post-treatment change, did not stratify patient risk, while sclerohyalinosis (>20%) singled out patients with the best outcome after neoadjuvant ChT. Clinical trial information: NCT01710176.