681 Background: Immune checkpoint inhibitors (ICI) are the standard of care for metastatic renal cell carcinoma (mRCC). Circadian rhythm drives organisms to properly predict and react to cyclical changes in the environment, and also affects the adaptive immune response. Here, we study the relationship between time-of-day ICI administration and outcomes in mRCC patients (pts). Methods: This is a single-center retrospective study of ICI-treated mRCC pts diagnosed from January 2014 to March 2022. Day and time of each ICI infusion (inf) for each pt were obtained from the pharmacy records. Proportion of ICI inf administered after 4:30 pm (ICI430) was calculated based on a prior publication (Quian, Lancet Oncol 2021). The primary outcome was overall survival (OS), and secondary endpoints were time on treatment (TOT), time to next treatment (TNT), and overall response rate (ORR). The proportion of ICI430 as a continuous variable, and the dichotomized data (≥20% and ≥50%) were correlated with OS, TOT and TNT by Kaplan-Meier analysis and Cox regression, and with ORR by logistic regression. Results: Overall, 104 pts and 1763 inf were analyzed (Table). 48 pts were treated with 1st line (1L) ICI. 903 inf were administered (median 12 inf per pt, 1 - 111), 146 (15.6%) after 4:30 pm. 12 (25%) and 2 (4.2%) pts received ≥ 20% and ≥ 50% ICI430. Due to the small number of events (5 deaths and 15 progressions), of ICI430, and heterogeneity of treatments in 1L, we focus on pts treated with 2nd line (2L) ICI. 56 pts were treated with ICI in ≥2L. 860 inf were administered (median 8 inf per pt, 1 - 123), 180 (20.9%) after 4:30 pm. 15 (34.1%) and 9 (20.5%) pts received ≥20% and ≥50% ICI430. Pts who received ≥20% after 4:30pm, received fewer inf (7 vs. 16, p=0.022), had a worse TOT (4.3 vs. 9 m, HR 2.5, p=0.013), a trend to a worse TNT (6.3 vs. 10.5 m, HR 1.9, p=0.06) and a worse OS (16.9 vs. 56.1 m, HR 3.1, p=0.01). Similar results were obtained when using ≥ 50% ICI430 as the cut-off (TOT, 2 vs. 9m, p=0.007; TNT 4.7 vs. 10.5m, p=0.05; OS 16.9 vs. 36.6m, p=0.1), as well as higher frequency of progressive disease (85.7 vs. 21.9%, p=0.006). A significant association with OS (HR 1.02, p=0.03) was shown when analyzing ICI430 as a continuous variable, meaning a 16% increase in the risk of death for each 10% increment of ICI inf after 4:30 pm. Conclusions: Administration of ≥2L ICI after 4:30 pm is associated with poor overall survival. Our results could have a direct impact on pt survival and organization of outpatient clinics, but further research is needed. [Table: see text]