e16518 Background: For patients with suspicious renal masses, it is often unclear which should receive an intervention versus initial surveillance. Both clear cell and papillary renal cell carcinoma overexpress kidney injury molecule-1 (KIM-1) due to their proximal epithelial cell of origin. We investigated whether plasma KIM-1 improves risk stratification of patients with suspicious kidney masses. Methods: Pre-nephrectomy plasma KIM-1 was measured in two independent cohorts of patients with renal masses. Cohort 1, from the prospective K2 trial, included 162 patients found to have ccRCC (cases) and 162 patients with benign renal masses (controls). Cohort 2 included 247 patients with small (cT1a) renal masses from an academic biorepository. Multivariable models were used to assess the relationship between pre-nephrectomy plasma KIM-1, surgical pathology, and subsequent clinical outcomes. Results: In cohort 1, plasma KIM-1 distinguished RCC versus benign masses with AUC-ROC 0.81 (95% CI: 0.76-0.86). In cohort 2 (cT1a renal masses), plasma KIM-1 distinguished RCC versus benign masses with AUC-ROC 0.74 (95% CI: 0.67 to 0.80). Among patients in cohort 2, the prevalence of malignancy was 97% among patients with the highest quartile of KIM-1 compared to 55% among patients with the lowest quartile of KIM-1. Plasma KIM-1 was higher in clear cell RCC and papillary RCC compared to other histologies (p<0.001 in each cohort). Cohort 1 patients had prospective long-term follow-up, and higher pre-nephrectomy KIM-1 was associated with worse metastasis free survival (multivariable MFS HR 1.29 per unit increase in log KIM-1, 95% CI 1.10-1.53) and worse overall survival (multivariable OS HR 1.31 per unit increase in log KIM-1, 95% CI 1.10-1.54). Conclusions: In two independent cohorts of patients with renal masses, elevated baseline plasma KIM-1 is associated with higher risk of malignant pathology, worse metastasis free survival, and higher risk of death. Plasma KIM-1 may be a useful risk stratification tool for patients with suspicious kidney masses.