The association between adult body mass index (BMI) and lung cancer (LC) susceptibility have been reported, but the causal relationship with childhood BMI remains largely unclear. To evaluate the causal effect of childhood BMI on LC susceptibility, a two-sample Mendelian randomization (MR) study was performed.The two-sample MR analysis utilized 25 single nucleotide polymorphisms (SNPs) as instrumental variables for childhood BMI. Genetic summary data from the International Lung Cancer Consortium and FinnGen databases were analyzed to estimate the causal effect of these SNPs on LC susceptibility. The IVW method was employed as the primary analysis, supplemented by the Weighted Median, MR-Egger, and MR pleiotropy residual sum and outlier test.Our findings indicated that there was no causal association between childhood BMI and the susceptibility of LC (odds ratio [OR]: 1.03, 95% confidence interval [CI]: 0.90-1.17, p = 0.705), lung adenocarcinoma (OR: 0.99, 95% CI: 0.86-1.13, p = 0.832), lung squamous cell carcinoma (OR: 0.97, 95% CI: 0.84-1.13, p = 0.726), and small cell LC (OR: 1.09, 95% CI: 0.82-1.45, p = 0.554) based on the IVW as well as other methods employed. Furthermore, these findings indicated no causal effect of childhood BMI on the LC susceptibility in both ever smokers and never smokers.This study did not conclude a causal effect between childhood BMI and LC susceptibility. However, given the complex nature of cancer development, further studies are needed to verify these findings.© 2023 The Authors. Cancer Medicine published by John Wiley & Sons Ltd.