Heart failure (HF) is a major problem in the long-term follow-up of adults with congenital heart disease (CHD) after cardiac surgery. The purpose of this study was to evaluate risk factors for HF in patients with CHD. N-terminal–pro-brain natriuretic peptide and maximal oxygen uptake (VO_2max) were measured in 345 consecutive patients with CHD. HF was defined as an elevated N-terminal–pro-brain natriuretic peptide level (≥100 pg/ml) and reduced VO_2max (≤25 ml/kg/min). The HF criteria were met by 89 patients. These patients were significantly older (mean ± SEM 30.8 ± 0.9 vs 24.8 ± 0.5 years), had significantly lower maximal heart rates (149 ± 3 vs 164 ± 1 beats/min), and had larger end-diastolic right ventricular diameters (36 ± 1 vs 27 ± 1 mm) and right ventricular pressure estimated by Doppler flow velocities of tricuspid valve regurgitation (2.9 ± 0.1 vs 2.3 ± 0.03 m/s). Mean fractional shortening of the left ventricle was within the normal range. To estimate risk stratification, odds ratios for HF were determined for the most frequently occurring types of congenital heart defects and surgical procedures. In conclusion, HF in adults with CHD predominately depends on diagnosis, age, the frequency of reoperation, and right ventricular function and may be related to chronotropic incompetence indicated by lower maximal heart rates.