To develop a new predictive equation for oxygen consumption (VO2) in children and adults with congenital and acquired heart disease.We retrospectively reviewed data from 502 consecutive patients (age 0-59 years) undergoing cardiac catheterisation with measured VO2 (M-VO2) and compared M-VO2 with VO2 from the LaFarge equations (LF-VO2) in patients <3 years (Group 1) and ≥3 years (Group 2). Factors associated with inaccurate LF-VO2 were used to develop a new predictive equation, which was prospectively validated in 100 consecutive patients (age 0-59 years).LF-VO2 was inaccurate in 42% of Group 1 (n=201) and 13% of Group 2 (n=301). Multivariable predictors of inaccurate LF-VO2 included age (OR 0.41, p=0.01) and single ventricle anatomy (OR 2.98, p=0.03) in Group 1 and anaemia (OR 0.84, p<0.001) in Group 2. Critical illness was borderline significant in both groups. The new predictive equation for VO2:[Formula: see text] Intraclass correlation between M-VO2 and the new predictive equation was good (r=0.53), whereas LF-VO2 was not (r=0.17). Bland-Altman analysis comparing M-VO2 with the new equation and with LF-VO2 demonstrated superiority of the new equation (mean bias 2.5 mL/min/m(2) vs -5.0 mL/min/m(2); limits of agreement -51.6, 56.5 vs -82.1, 72).VO2 derived from the LaFarge equations is frequently inaccurate, particularly in younger patients, and will lead to erroneous haemodynamic calculations. We developed and prospectively validated a new VO2 predictive equation for use in patients of all ages with congenital and acquired heart disease.