Fast breathhold cardiovascular magnetic resonance (CMR) has become a reference standard for the measurement of cardiac volumes, function, and mass. The implications of this for sample sizes for remodeling studies in heart failure (HF) have not been elucidated. We determined the reproducibility of CMR in HF and calculated the sample size requirements and compared them with published values for echocardiography. Breathhold gradient echo cines of the left ventricle were acquired in 20 patients with HF and 20 normal subjects. Sample size values were calculated from the interstudy standard deviation of the difference. The percentage variability of the measured parameters in our HF group of intraobserver (2.0-7.4%), interobserver (3.3-7.7%), and interstudy (2.5-4.8%) measurements was slightly larger than for our normal group (1.6-6.6%, 1.6-7.3%, and 2.0-7.3%, respectively) but remained comparable with previous studies in normal subjects. The calculated sample sizes in patients with HF for CMR to detect a 10-ml change in end-diastolic volume (n = 12) and end-systolic volume (n = 10), a 3% change in ejection fraction (n = 15), and a 10-g change in mass was (n = 9) were substantially smaller than recently published values for two-dimensional echocardiography (reduction of 81-97%). Breathhold CMR is a fast comprehensive technique for the assessment of cardiac volumes, function, and mass in HF that is accurate but also highly reproducible. This allows a considerable reduction in the patient numbers required to prove a hypothesis in research studies, which suggests a potential for important research cost savings.