Background:　The　relationship　between　body　composition　and　hematological　malignancies　is　poorly　understood.　Using　mendelian　randomization　(MR)　analysis,　this　study　aimed　to　assess　the　genetic　associations　between　body　composition　and　hematological　malignancies.　Methods:　Data　from　the　UK　Biobank　Genome-Wide　Association　Studies　database,　which　includes　approximately　500,000　participants　aged　40–69 years,　were　utilized.　Multivariable　MR　analysis　and　the　inverse-variance　weighted　(IVW)　method　were　employed　to　assess　the　causal　link　between　exposures　and　outcomes.　Sensitivity　analyses　were　performed　to　evaluate　the　heterogeneity　and　pleiotropy　of　the　instrumental　variables.　Results:　The　univariable　MR　analysis　revealed　that　specific　body　composition　parameters,　including　arm　fat-free　mass　(left　and　right),　trunk-predicted　mass,　whole-body　fat-free　mass,　and　whole-body　water　mass,　were　associated　with　an　increased　leukemia　risk.　Arm　fat-free　mass　(right)　and　fat　mass　(left　and　right);　leg　fat-free　mass　(left　and　right)　and　fat　mass　(left　and　right);　trunk　fat-free　mass,　fat　mass,　and　predicted　mass;　and　whole-body　fat-free　mass,　fat　mass,　and　water　mass　were　associated　with　an　increased　lymphoma　risk.　However,　no　causal　relationship　was　observed　between　body　composition　parameters　and　multiple　myeloma.　In　the　multivariable　MR　analysis,　height　[odds　ratio　(OR)　=　1.004,　p　=　0.040]　was　identified　as　an　independent　risk　factor　for　lymphoma,　while　the　waist-to-hip　ratio　(OR　=　1.003,　p　=　0.004)　increased　the　risk　of　multiple　myeloma.　Conclusion:　Height　increases　the　risk　of　lymphoma,　while　the　waist-to-hip　ratio　is　a　risk　factor　for　multiple　myeloma.　These　findings　offer　further　evidence　supporting　a　causal　relationship　between　body　composition　and　hematological　malignancies.　©　The　Author(s),　under　exclusive　licence　to　Springer　Nature　Switzerland　AG　2025.