In 20 subjects with chronic congestive heart failure due to coronary artery disease, the 24-hour variability of ambulatory intraarterial blood pressure (BP) was studied using an improved Oxford Medilog system, and correlated with left ventricular function at rest. The mean radionuclide ejection fraction was 27% (range 10 to 42), the mean pulmonary arterial wedge pressure was 18 mm Hg (5 to 37) and the mean cardiac index was 2.8 liters/min/m2 (2 to 3.8). The 24-hour systolic BP and heart rate (HR) variability indexes were less than those of 22 normal volunteers (p less than 0.05) and were strongly correlated (p less than 0.05) with ejection fraction at rest and pulmonary arterial wedge pressure. Stepwise regression showed that a combination of the mean nocturnal HR and the standard deviation of the hourly mean systolic BP values accounted for 67% of the variability in ejection fraction between patients. Similarly, 73% of the variation in pulmonary wedge pressure was explained by combining the 24-hour mean HR and the mean nocturnal HR.