ObjectiveA two-sample Mendelian randomization analysis was used to explore the causal associations between four basic body indices (basal metabolic rate, body fat percentage, BMI and hip circumference) and myasthenia gravis (MG). MethodsPooled gene-wide association study (GWAS) data were obtained from large publicly searchable databases, and four basic body indices were selected as the exposure factors and myasthenia gravis as the outcome factors, and single nucleotide polymorphisms (SNPs), which were strongly correlated with the phenotype of the exposure factors, were screened as the instrumental variables, and two-sample Mendelian randomization analyses were performed in order to assess the potential causal relationship between the exposure and the disease. ResultsInverse variance weighting (IVW) analysis showed that increased basal metabolic rate (OR=1.39, 95%CI 1.00 to 1.93, P=0.047), body fat percentage (OR=1.61, 95%CI 1.06 to 2.44, P=0.024), and hip circumference (OR=1.67, 95%CI 1.29 to 2.17, P<0.001) increased the risk of MG. But there was no significant causal relationship between BMI and MG. ConclusionBasal metabolic rate, body fat percentage and hip circumference have a positive causal relationship with MG, while BMI does not have a significant causal relationship with MG.