PURPOSE: The aim of the INSTINCT trial was to assess the effect of intravenous polyspecific immunoglobulin G (IVIG) compared with placebo on self-reported physical function in intensive care unit (ICU) patients with necrotising soft tissue infection (NSTI). METHODS: We randomised 100 patients with NSTI 1:1 to masked infusion of 25 g of IVIG (Privigen, CSL Behring) or an equal volume of 0.9% saline once daily for the first 3 days of ICU admission. The primary outcome was the physical component summary (PCS) score of the 36-item short form health survey (SF-36) 6 months after randomisation; patients who had died were given the lowest possible score (zero). RESULTS: Of the 100 patients randomised, 87 were included in the intention-to-treat analysis of the PCS score, 42 patients (84%) in the IVIG group and 45 patients (90%) in the placebo group. The two intervention groups had similar baseline characteristics with the exception of IVIG use before randomisation (1 dose was allowed) and rates of acute kidney injury. Median PCS scores were 36 (interquartile range 0-43) in the group assigned to IVIG and 31 (0-47) in the group assigned to placebo (mean adjusted difference 1 (95% confidence interval -7 to 10), p = 0.81). The result was supported by analyses adjusted for baseline prognostics, those in the per protocol populations, in the subgroups (site of NSTI) and those done post hoc adjusted for IVIG use before randomisation. CONCLUSIONS: In ICU patients with NSTI, we observed no apparent effects of adjuvant IVIG on self-reported physical functioning at 6 months. TRIAL REGISTRATION: NCT02111161.