: Poor cardiorespiratory fitness (CRF) is an independent predictor of cardiovascular and all-cause mortality. Yet, our knowledge about the prevalence of poor CRF is limited. We investigated the prevalence of poor CRF among middle-aged Danish adults from a community-based health promotion program, including identification of a subgroup with high cardiovascular mortality risk. This cross-sectional study included 2,253 middle-aged adults, who completed a preventive health check including CRF testing. CRF (ml O 2 /min/kg) was assessed using the Astrand-Ryhming cycle ergometer test. High 10-year cardiovascular mortality risk as defined in the Danish 2016 guidelines was assessed using questionnaires, health examinations, and prescription data from the Danish National Prescription Registry. The prevalence of poor CRF was 51.7% (95% confidence interval [CI] 48.7-54.7) among men and 31.3% (95% CI 28.7-34.1) among women. A total of 216 (19.4%) men and 220 (19.3%) women were identified with a high 10-year cardiovascular mortality risk. Among these, 65.0% of men (95% CI 58.2-71.3) and 44.1% of women (95% CI 37.4-50.9) had poor CRF. Half of men and one third of women participating in a community-based health promotion program were identified with poor CRF. Among high-risk individuals, two thirds of men and almost every second woman had a poor CRF. Our results emphasise the need for effective public health strategies and interventions to increase CRF in the general population and among high-risk individuals in particular in order to improve public health and reduce mortality.