Simple SummaryCurrently, clinical studies exploring the impact of high body fat on toxicities after receiving immune checkpoint inhibitors (ICIs) among cancer patients are limited. Here, we analyze data from a health care system serving the mid-Atlantic geographic region to assess how body fat can affect the development of toxicities of ICIs. In our study, body mass index (BMI) was used as the measure of body fat, and the results suggested that cancer patients with a high BMI were more likely to have toxicities after receiving ICIs. Our study suggests that symptom management should be incorporated in the cancer care continuum of patients who receive ICIs, especially those with high BMI. In clinical settings, oncologists should inform cancer patients receiving ICIs with high BMI that their risk of post-treatment toxicities can be higher compared to their counterparts with lower BMI.Evidence regarding the association between body mass index (BMI) and immune-related adverse events (irAEs) among cancer patients receiving immune checkpoint inhibitors (ICIs) is limited. Here, we use cross-sectional hospital-based data to explore their relationship. Pre-treatment BMI was treated as an ordinal variable (<25, 25 to ≤30, ≥30 kg/m2). The outcome of interest was irAEs after ICI initiation. A multivariable logistic regression model estimated the adjusted odds ratio (aOR) and 95% confidence interval (CI) of BMI. A total of 684 patients with stage III or IV cancer were included in the study (lung: 269, melanoma: 204, other: 211). The mean age at the first dose of ICI was 64.1 years (SD = 13.5), 394 patients (57.6%) were male, and over one-third (N = 260, 38.0%) were non-White. Overall, 52.9% of patients had BMI ≥ 25 kg/m2 (25 to ≤30: 217, ≥30: 145) and 288 (42.1%) had irAEs after ICI treatment. Patients with higher BMI tended to have a higher rate of irAEs (<25: 35.7%, 25 to ≤30: 47.0%, ≥30: 49.0%). The multivariable logistic regression yielded consistent results (BMI ≥ 30 vs. BMI < 25: aOR = 1.47, 95% CI = 0.96–2.23; 25 ≤ BMI < 30 vs. BMI < 25: aOR = 1.46, 95% CI = 1.02–2.11, p-trend = 0.04). In conclusion, among patients with advanced cancer receiving ICIs, the rate of irAEs appears to be higher among those with higher BMI.