620 Background: Existing data suggest that percutaneous renal mass ablation offers favorable rates of disease control with lower cost and fewer complications relative to surgical interventions. Despite increasing utilization, little is known about changes in practice patterns and patient selection over time as new technologies have emerged and indications for renal ablation have been refined. Methods: Institutional review board approval was granted with a waiver of informed consent to review data from the multinational Ablation of Renal Masses Outcomes Registry (ARMOR), which is composed of adult ablation patients with biopsied renal masses. Patient demographics, ablation modality, timing of renal biopsy, malignancy rate, method of anesthesia, and type of imaging guidance from 2001-2016 were reviewed. Analyses were conducted using generalized mixed modeling assuming normal or binary distribution with sandwich estimation, where observations were nested within participating institutions. Variables were modeled over time using SAS/GLIMMIX with significance set at α = 0.05. Results: Data from 768 patients at eleven institutions were available for review. Average patient age was 67.7±11.7 years (male:female, 1.9:1), which remained unchanged throughout the study period. Radiofrequency ablation (RFA) was the most commonly utilized modality from 2001-2007 but decreased thereafter, with cryoablation favored from 2008-2012. By 2016, RFA, microwave (MWA), and cryoablation were all used with relative equipoise, though a significant trend toward increased MWA utilization was found beginning in 2010 (OR: 1.28; p = 0.004). A significant increase in the frequency of same-day biopsy was observed relative to pre-ablation biopsy (OR: 1.16; p = 0.003) alongside a trend toward increasing rates of benign histology (p = 0.08). Use of CT guidance and general anesthesia increased by 4% and 5%, respectively. Conclusions: ARMOR data illustrate fluctuating practice patterns, with increasing use of microwave ablation technology. Growing utilization of concurrent renal mass biopsy and ablation may be contributing to the observed trend toward increased rates of ablation for benign tumors.