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Background We tested the hypothesis that genetically high body mass index (BMI) and genetically high tobacco smoking jointly are causally associated with a higher risk of chronic obstructive pulmonary disease (COPD) than each risk factor alone.Methods We used two large Danish population-based cohort studies, The Copenhagen City Heart Study and the Copenhagen General Population Study linked to national Danish patient discharge and death registries. For the genetic analysis, we included 112 943 adults genotyped for five BMI and one tobacco smoking increasing gene variant. Risk ratios with 95% CIs for COPD were estimated for the risk factors in combination and individually by use of allele scores for BMI and smoking.Results Separately, both high BMI and tobacco smoking increased the risk for COPD (Ps<0.001), yet the highest risk was observed in the group of individuals with a combination of high allele scores for both high BMI and tobacco smoking (Ps≤0.04). Observationally, the highest risk for COPD was observed in the groups of people having high cumulative use of tobacco smoking, irrespective of BMI.Conclusions The combination of causal, genetically high BMI and tobacco smoking resulted in a higher risk for COPD than the two risk factors alone.