e21039 Background: Tumor-infiltrating neutrophils (TINs) and their myeloid precursors play an important role in cancer biology. While activated neutrophils have been shown to kill tumor cells, there is growing evidence that neutrophils may also support tumor progression by impairing T cell-dependent anti-tumor immunity. We hypothesized that low TINs would be associated with better responses to immunotherapy (IO). Methods: Pretreatment biopsies from 15 metastatic melanoma (MM) pts treated with anti-CTL4 and anti-PD-1 therapy from 2012-17 were collected and stained with myeloperoxidase (MPO). Slides were scanned at 40x magnification on the Aperio ScanScope AT Turbo brightfield instrument (Leica Biosystems) at a resolution of 0.25 microns per pixel. To indicate the region of analysis, the digital pen tool was used to trace a minimum 85% of tumor. Ten fixed-sized boxes were placed on the hottest staining region in a second annotation layer on the image. The boxes equaled 1mm2 in total area. If the tumor tracing had an area of 1mm2, or less, boxes were not placed on the image. The selected tissue was analyzed using an Aperio imaging quatification algorithm . The number of 3+ cells were considered positive for neutrophils and used in subsequent calculations. Quality control review was performed on a subset of cases by Anatomic Pathologists. Response to therapy was assessed using the Response Evaluation Criteria In Solid Tumors (RECIST). Overall survival (OS) was calculated using the Kaplan Meier method. Results: The median number of treatment lines before the biopsy was 1 (range 0-2). The treatment immediately following the biopsy consisted of ipilimumab (n = 10, 66.6%), pembrolizumab (n = 4, 26.6%), ipilimumab/nivolumab (n = 1, 6.6%). The median number of neutrophils in the biopsy hotspot was 92/mm2 (range 6-219) for the entire cohort. For patients achieving PR and CR after 4 cycles of therapy, the median neutrophils were 36/mm2 (IQ 14-78) while the median was 106/mm2 (IQ 60-138) for patients who had progressive disease,( p = 0.04). There was no correlation between the numbers of neutrophils in the biopsy with the absolute neutrophil count in the peripheral blood (R value: 0.012). Conclusions: Decreased TINs are significantly associated with better responses to IO in MM pts. Measurements of TINs may help to select patients who are likely to benefit from anti-CTL4 and anti-PD-1 therapy. Additionally, exploration of whether combining therapies that decrease neutrophils intratumorally should be explored.