Background: The phase 3 JAVELIN Renal 101 trial in patients (pts) with advanced RCC demonstrated a progression-free survival (PFS) benefit and higher objective response rate (ORR) with avelumab + axitinib (A+Ax) vs sunitinib (S) (Motzer NEJM 2019). PFS and ORR favored A+Ax in all MSKCC risk groups, but median PFS varied. Here, we report results from analyses of baseline tumor samples to define molecular characteristics underlying risk group classifications. Methods: Nephrectomy or tumor samples from pts enrolled in the study were characterized by immunohistochemistry (CD8 and PD-L1), whole exome sequencing, or gene expression profiling (n = 705–850). Gene expression signatures (GES), pathway activation status, and mutational profiles were examined in relation to MSKCC risk groups. Results: Of the 886 total pts enrolled, 23%, 66%, and 11% had favorable (F), intermediate (I), or poor (P) MSKCC risk factors at baseline. In the F, I, and P groups, the ORR was 66%/38%, 50%/24%, and 31%/9%; median PFS was NR/16.7 mo, 13.3 mo/7.9 mo, and 5.6 mo/2.8 mo for the A+Ax/S arms, respectively. Neither the presence of PD-L1+ immune cells nor CD8+ cells differentiated the subgroups; however, the presence of PD-L1+ tumor cells was highest in the P group (p=0.0159). When compared to the I and P groups, the F group was enriched for NOTCH2 mutations (p=0.0002), displayed high FLT1 expression (p=0.007), and showed a trend favoring angiogenesis GES (JAVELIN Renal 101 and IMmotion150). The I group displayed few distinguishing characteristics (low neutrophil GES [p=0.02] and elevated homeobox gene expression [p=0.00052]). Relative to the F group, the P group showed higher cell cycle gene expression (p=0.0057) and PTEN mutation frequency, wild-type NOTCH2 genotype, elevated IMmotion150 Myeloid inflamed GES (p=0.0024), and macrophage-specific GES (p=0.0327), as well as GES for TH2 (p=0.0002), hypoxia (p=0.0199), glycolysis (p=0.0066), and the lowest expression of a dendritic cell GES (p=0.0209). Conclusions: In advanced RCC patients, biological differences within each MSKCC risk category may impact response to treatment and may help explain why these groups perform differently. Clinical trial information: NCT02684006