Objective To investigate the causal association between bronchial asthma and bone mineral density at different sites using a two-sample Mendelian randomization (MR) approach. Methods Summary data for exposure factors and outcome were obtained from different genome-wide association studies.Single nucleotide polymorphisms strongly associated with bronchial asthma were selected as instrumental variables,and those in linkage disequilibrium were excluded.The inverse-variance weighted (IVW) method was used as the primary method for MR analysis,complemented by weighted median,simple mode,weighted mode,and MR-Egger regression methods.Sensitivity analyses were conducted to assess the stability of the results. Results The random-effects model of IVW analysis showed that heel bone mineral density (OR=0.986;95% CI,0.974 to 0.998;P=0.023) as the outcome dataset had a reverse causal effect with bronchial asthma,while lumbar spine bone mineral density (OR=1.031;95% CI,0.984 to 1.081;P=0.195),femoral neck bone mineral density (OR=1.014;95% CI,0.973 to 1.057;P=0.505),and forearm bone mineral density (OR=1.011;95% CI,0.935 to 1.094;P=0.775) as outcome datasets showed no causal effect with bronchial asthma.The MR-Egger intercept test results indicated that the P-values for the intercepts of lumbar bone mineral density,femoral neck bone mineral density,forearm bone mineral density,and calcaneal bone mineral density were all over 0.05,suggesting no horizontal pleiotropy and relatively stable results. Conclusion MR analysis reveals a reverse causal effect between bronchial asthma and heel bone mineral density,suggesting that clinicians should strengthen the monitoring of heel bone mineral density in patients with bronchial asthma to timely detect and intervene osteoporosis.