Background: Anti-PD1 antibodies (Abs) Nivolumab (Nivo) and Pembrolizumab (Pembro) have both demonstrated to improve the overall survival (OS) in patients (pts) with advanced or metastatic melanoma (MM) in the first line setting. We previously reported that an increase in absolute eosinophil counts (AEC) of 100/mm3 over baseline at week 3 and an AEC > 400/mm3 at week 12 during anti-PD1 treatment (tmt) might identify pts with MM most likely to experience long-term disease control ⁽¹⁾. AEC could serve as an important marker allowing the detection of those patients who may benefit from the therapy with anti-PD1 Abs. Methods: This retrospective observational study included pts with MM who received anti-PD1 tmt in a single institution. The objective was to identify whether an increase over the upper limit of normal AEC (>400/mm3) at any time during the tmt could predict better outcomes. Blood tests were performed before every tmt administration. Progression free survival (PFS) and OS were evaluated. Descriptive statistics were used to analyze patient baseline characteristics. PFS and OS were estimated by the Kaplan-Meier method. Results: From March 2013 to February 2016, 50 pts were treated with anti-PD1 Abs (39 pts with Pembro and 11 pts with Nivo). Median age was 58 years (33-84). 72% had received previous systemic tmt for MM, 64% had stage M1c disease, and 50% had elevated LDH levels. BRAF V600 mutations were observed in 34%. The median time to increase AEC > 400/mm2 was 42 (14-196) days. Pts who experienced an increase > 400/mm3 at 12 weeks demonstrated better outcomes in terms of PFS (21,9 [95% CI: 11,1-32,6] vs. 7,5 [95% CI: 4,3-10,7] months, p=0.017) and OS (38,0 [95% CI: 30,8-45,1] vs. 20,5 [95% CI: 15,7-25,3] months, p=0.025) compared with those who did not. Moreover, according to our previous results, pts with an increase of at least 100/mm3 in AEC over the baseline at 12 weeks also showed better OS (32,5 [95% CI: 26,3-38,8] vs. 16,3 [95% CI: 11,1-21,6] months, p=0.008). Conclusions: An increase in AEC of 100/mm3 over baseline and an absolute AEC > 400/mm3 at week 12 during anti-PD1 tmt might identify pts with MM most likely to experience clinical benefit with anti-PD1 Abs which could be relevant for pts management.