The aim of this study was to provide decision makers with an assessment of potential future health effects of interventions against overweight and obesity (OWOB). By means of the DYNAMO-HIA tool we conducted a health impact assessment simulating future prevented disease (ischemic heart disease (IHD), diabetes, stroke, and multi morbidity) incidence, prevalence and life expectancy (LE) related to a scenario where OWOB is reduced by 25% and a scenario where obesity is eliminated. The study covered projected number of persons living in Copenhagen, Denmark during year 2014-2040 (n 2040 = 742,129). Reducing the proportion of men/women with OWOB with 25% will increase population LE by 2.4/1.2 months and at the same time decrease LE with diabetes by 3.1/2.2 months. As a result of eliminating obesity, total LE will increase by 6.0/3.6 months and LE with diabetes will decrease with 9.8/10.3 months for men/women. We found no important effects on LE with IHD and stroke. This illustrates that the positive effects of lowering OWOB levels on IHD and stroke incidence is offset due to increasing total LE. Although the population of Copenhagen is relatively lean, reducing obesity levels will result in significant benefits for population cardiometabolic health status and LE. Future public health prevention programs may use the results as reference data for potential impact of reductions in OWOB.