Aim: The study aimed to find out whether there is a relationship between the mononeuropathies of the median, ulnar, radial, peroneal, and sciatic nerves and body mass index (BMI). Material and Methods: Patients whose clinical and electrodiagnostic findings were compatible with carpal tunnel syndrome (CTS), ulnar neuropathy at the elbow (UNE), radial neuropathy at the spiral groove (RNS), peroneal neuropathy at the fibular head (PNFH), and sciatic injury due to intramuscular injection (SNIII) were included in this retrospective cohort study. In addition, controls whose clinical and electrodiagnostic features were not compatible with mononeuropathy were included in the study. The BMI values of all participants were analyzed. Results: One hundred thirty-one CTS patients, 53 UNE patients, 6 RNS patients, 25 PNFH patients, 72 SNIII patients, and 53 controls were included in the study. The BMI of CTS patients was higher than the BMI of controls (p<0.001), PNFH patients (p<0.001), and SNIII patients (p<0.001). The BMI of SNIII patients was lower than the BMI of controls (p<0.001), CTS patients (p<0.001), and UNE patients (p<0.001). The BMI of PNFH patients was lower than that of CTS patients (p<0.001) and UNE patients (p=0.004). No significant correlation was found between BMI values and electrodiagnostic classification of mononeuropathies in the groups. Conclusion: This study showed that high BMI is a risk factor for CTS and low BMI is a risk factor for SNIII. There may also be a relationship between BMI and PNFH, but this should be confirmed by further studies.