A retrospective cohort study using the data from The Health Improvement Network (THIN) database in the United Kingdom was conducted to examine the incidence rates of seizures across different BMI levels in the adult population aged ≥18 years. Poisson regression was used to examine the relationship between BMI and seizures. The overall incidence rate of seizures was found to be 31.2 cases per 100,000 person-years. The incidence rate of seizures (cases per 100,000 person-years) in obese patients (BMI ≥ 30 kg/m2) was 34.8 (95% confidence interval (CI), 23.1, 46.4), comparable to that in patients with normal weight (BMI between 18.5 and 24.9 kg/m2) (35.8, 95%CI (26.6, 44.9)). In contrast, underweight patients (<18.5 kg/m2) or extremely obese (≥40 kg/m2) patients tended to have higher incidence rates than those with normal weight. After adjustment for age, gender, and smoking status, compared to patients with normal weight, those who were underweight or extremely obese had a rate ratio (RR) for seizures of 1.6 (95%CI (0.7, 3.8)) and 1.7 (95%CI (0.7, 3.9)), respectively. To date, we have not found any study that examines the associations between BMI or obesity and seizures. In this study, the incidence rates of seizures in the extremely obese and underweight patients tended to be higher than that in the normal-weight patients.