Increased body mass index (BMI) has been associated with poor outcomes in solid malignancies. However, a recent study showed that higher BMI was associated with improved overall survival (OS) in advanced non-small lung cancer (NSCLC) treated with atezolizumab. Whether the obesity paradox applies to HIV patients with advanced NSCLC is unknown, as HIV patients were generally excluded from previous immunotherapy trials. We conducted a retrospective study on HIV patients with advanced NSCLC undergoing treatment with immunotherapy from 9/1/2016 to 3/01/2020 at a NCI-designated cancer center in Bronx, New York. Information was collected including age, race, ethnicity, BMI, PD-L1 tumor proportion score (TPS), neutrophil-to-lymphocyte ratio (NLR), HIV viral load (VL), and CD4/CD8 count at therapy initiation. Patients were categorized into underweight (<18.5 kg/m2), normal (18.5-24.9 kg/m2), and overweight/obese BMI (≥ 25 kg/m2) per World Health Organization (WHO) categories. Data was collected until date of death, date of last contact, or censored after 3/1/20. The primary outcome was OS, and the secondary outcome for BMI was progression free survival (PFS). Cox proportional hazards was used to examine the association of BMI, NLR, and PD-L1 TPS with survival. Sixteen patients were identified who had HIV and received immunotherapy for treatment of advanced NSCLC. The mean age was 61.3 years. At treatment initiation, 9 had undetectable VL and 4 had detectable but unquantifiable VL, the mean CD4 count was 469, and the mean CD4/CD8 ratio was 0.69. Five patients were underweight, 4 were overweight/obese, and 7 patients had a normal BMI. Compared to patients with normal BMI, underweight patients had significantly worse OS (HR 12.3, 95% CI 2.02-74.5, p=0.006), and overweight/obese patients had similar OS (HR 0.93, 95% CI 0.17-5.22, p>0.9). PFS was not significantly different for underweight (HR 1.51, 95% CI 0.33, 6.86, 0.6) or overweight/obese (HR 1.88, 95% CI 0.41-8.63, p=0.4) patients. Patients with NLR ≥5 had a trend towards inferior OS (HR 3.39, 95% CI 0.96-12, p=0.057). PD-L1 TPS was available in 15 patients and was not a significant factor in OS at the >50% cutoff (HR 0.83, 95% 0.21-3.35, p=0.8). In our cohort of HIV patients with advanced NSCLC undergoing immunotherapy, compared to patients with normal BMI, underweight patients were observed to have significantly inferior OS. We acknowledge that the major shortcoming of current study is the small sample size, but even with this limited sample size, this finding was significant with a p-value of 0.006. Conversely, it is also possible that factors with borderline p-values, such as NLR, could also be significant in larger cohorts, and further investigation should continue.