Background: In the phase 3 TOWER study (NCT02013167), blinatumomab (BLIN) significantly improved overall survival in adults with Ph- R/R BCP ALL, and these patients (pts) also reported better HRQoL (EORTC QLQ-C30 measures) compared with standard of care (SOC) chemotherapy. This analysis assessed the impact of grade 3+ infection, which was reported in 34.1% of BLIN pts vs 52.3% of SOC pts, on HRQoL in TOWER. Methods: Pts were stratified into two groups: those with infection vs those without infection, between baseline and Day 8, Day 15, and Day 29. A difference-in-difference (DID) model investigated the impact of infection on HRQoL. A 5-point change is considered the minimum clinically important difference in the EORTC QLQ-C30 for between group comparisons (King et al. 1996). Results: In total, 342 pts (n = 247 BLIN; n = 95 SOC) had a non-missing baseline and ≥1 post-baseline HRQoL score for any scale. Grade 3+ infection was reported in 15 (4.5%) pts before Day 8, 49 (14.5%) pts before Day 15, and 76 (22.6%) pts before Day 29. Pts with grade 3+ infection reported clinical meaningful deterioration in global health status (GHS) score and in almost all functional subscales across timepoints assessed (Table). At Day 29, grade 3+ infection was associated with statistically significant and clinically meaningful deterioration in GHS and most functional scales (Table). Similar but smaller effects were also observed for symptom scales/items (data not shown). Conclusions: Grade 3+ infection in pts with Ph- R/R BCP ALL was associated with clinically meaningful deterioration in HRQoL. The lower incidence of infection in pts treated with BLIN vs SOC may have contributed to the improved HRQoL for pts treated with BLIN. Clinical trial information: NCT02013167