Abstract Funding Acknowledgements Type of funding sources: None. Background The Myocardial contraction fraction (MCF% – stroke volume over myocardial volume) is an alternative/complementary measure of cardiac function to the ejection fraction (EF% – stroke volume over end diastolic volume), (1–2). It is dimensionless, does not need allometric scaling, is easy to compute and intuitive: the stroke volume produced per unit of myocardium, with some data for its utility in different forms of hypertrophy, (3–5). EF limitations (eg HFpEF) are well known. We explored the potential utility of MCF in a variety of cardiac diseases. Purpose To define reference ranges for MCF in normal and disease using a large dataset, exploring sex and age-related differences. To evaluate and compare the potential use of MCF as a prognostic biomarker compared to the more commonly used LVEF. Methods 3 cohorts were used: 1. Health: 3,541 subjects from UK biobank, supplemented by 72 local healthy volunteers (UKB lower age limit is 45) to define normality and age/sex differences. 2. Disease: n=380 with a range of diseases : 125 HCM, 157 Fabry disease (FD) and 26 Transthyretin (TTR) Amyloid. 3. Prognostic cohort: n=1500 patients from a single centre with outcomes (26% death or hospitalisation with heart failure over a median 5.5years follow-up) All CMR images were analysed using a clinically validated artificial intelligence (AI) algorithm for convenience of large-scale analysis (6) to determine MCF and LVEF. Results Health: There was no significant changes in MCF with age but there was a significant sex difference (male 0.94, range 0.68–1.2; female 1.10, range 0.82–1.37 p<0.05)—see figure 1. Disease: MCF values for each disease cohort is shown in figure 2, all with a significant reduction compared to the healthy controls. Values ranged from 0.47 with Amyloid to 0.94 with Fabrys. Outcome: MCF strongly predicted patient outcomes – and was a more powerful predictor than LVEF (LVEF chi-squared=84 vs MCF chi-squared=160). Conclusion MCF is a complementary measure of cardiac performance to LVEF and equally as logical, intuitive and easy to calculate. It is frequently reduced even when LVEF is normal, and is a much stronger predictor of outcome than LVEF.