Background: The benefit of vascular endothelial growth factor tyrosine kinase inhibitors (VEGF-TKIs) as adjuvant therapy for RCC remains a subject of controversy. Given the uncertain risk-benefit ratio, identifying subgroups expected to derive benefit or harm from therapy could lead to a more precise approach to therapy. Because of known sex-differences in the pharmacokinetics and tissue distribution of VEGF-TKIs, we assessed the interaction of age and sex on treatment outcomes among patients in the phase III ASSURE trial (Adjuvant Sorafenib or Sunitinib for Unfavorable RCC). Methods: ASSURE included 1,943 patients with ≥ pT1b resected RCC. There was no OS or DFS benefit with adjuvant sunitinib or sorafenib relative to placebo. In this post-hoc subgroup analysis of age and sex based on the median age of 56 years, Cox regression computed hazard ratios (HR) and 95% confidence intervals (CIs) for OS and DFS with sunitinib or sorafenib compared to placebo across age- and sex-specific strata (males ≤ 56 years [n = 689], males > 56 years [n = 620], females ≤ 56 years [n = 317], and females > 56 years [n = 317]). Models were adjusted for race, histology, and UISS risk group. A 3-way interaction term was calculated for sex, age, and treatment. Results: Treatment with sunitinib was associated with increased mortality among women > 56 years (sunitinib vs placebo [HR _OS 2.38; 95% CI, 1.39-4.07]), but not in women ≤ 56 years or in men of any age. Similarly, among women > 56 years, the risk of disease recurrence was increased with sunitinib relative to placebo (HR _DFS 1.53; 95% CI, 1.03-2.28). There were no statistically significant differences in OS or DFS associated with sorafenib (women > 56 years: HR _OS 1.50, 95% CI, 0.83-2.71; HR _DFS 1.23, 95% CI, 0.81-1.89). The 3-way interaction between age, sex, and sunitinib treatment on mortality was statistically significant (p = 0.0166). Conclusions: Adjuvant sunitinib after nephrectomy was associated with increased mortality among older women, which may be explained by increased risk of RCC recurrence. These findings, if validated, highlight the importance of considering sex and age on VEGF-TKI treatment outcomes, future study designs, and RCC tumor biology. Clinical trial information: NCT00326898