Background: Obesity is linked to an increased risk of cancer development. The excess of body fatness seems to be associated with alterations in hormonal, metabolic and inflammatory pathways, that may lead to activation of the carcinogenesis process. Previous studies suggested this obesity pro-inflammatory state could improve ICI clinical efficacy. Methods: Baseline characteristics and clinical outcomes were retrospectively collected from advanced cancer patients of any primary site, and treated with ICI in our institution. The BMI was determined for all pts and categorized into 2 groups: obese (BMI≥30) and non-obese (BMI < 30). Primary outcomes were the association of BMI category with overall survival (OS) and progression free survival (PFS) assessed by log-rank statistic, and both were stratified by sex, age, treatment agent and primary tumor site using the Cox-regression. Secondary outcome was the association of BMI with objective response rate (ORR). Results: We collected data from 448 advanced cancer pts - 192 (43%) as normal weight, 159 (36%) as overweight, 78 (17%) as obese and 19 (4%) as underweight. A total of 370 pts (83%) were included in the non-obese group (BMI < 30) and 78 (17%) patients in the obese group (BMI≥30). The majority of pts (387 - 84%) received anti-PD-1/anti-PD-L1; 128 pts (28%) were treated at 1st line and 200 pts (44%) at 2nd line. The obese group experienced longer mOS than the non-obese group - 21.8 months (95% CI NR - NR) vs. 14.9 months (95% CI 8.3 -21.5); HR = 0.82, (95% CI 0.57-1.18, P = 0.28). However, this was not statistically significant and even after stratification. The obese group had an inferior mPFS than the non-obese group - 4.7 months (95% CI 3.8- 5.7) vs. 5.3 months (95% CI 3.45-7.15); - HR = 0.99, (95% CI 0.76 -1.30), P = 0.95. There was no significative difference in mPFS and ORR according to BMI. Conclusions: Although study did not report an improved OS among high BMI pts treated with ICI, our results suggested a trend in survival benefit. The BMI should be explored as a stratification variable in the design of prospective trials with advanced cancer pts and ICI treatment.