Low blood pressure in the very old has been associated with organ dysfunction and excess mortality but the underlying mechanism has yet to be elucidated. We hypothesized that cardiac dysfunction contributes to low blood pressure in the very old. We invited a convenience sample consisting of 82 participants all aged 90 years from a population-based cohort study in the very old. Blood pressure was measured twice, and all but one underwent echocardiography to assess cardiac dimensions and functional cardiac parameters. Some 47 participants were free from haemodynamically significant valvular disease and were included in the present analyses. There were low values for mean cardiac output (2.04 l(-1) min(-1) m(-2), s.e. 0.40) and mean stroke volume (31.4 ml m(-2), s.e. 7.7). For every 10-mm Hg decrease in systolic blood pressure, cardiac output was 0.09 l(-1) min(-1) m(-2) lower (s.e. 0.04, P=0.019), and stroke volume was 1.58 ml m(-2) lower (s.e. 0.68, P=0.024). Mean left ventricular ejection fraction was normal and 2.39% (s.e. 1.16, P=0.046) higher for each 10-mm Hg decrease in systolic blood pressure. Mean left ventricular dimensions were normal but the E/A ratio was reduced (0.68, s.d. 0.21), indicating diastolic dysfunction. In conclusion, among the oldest old, low systolic blood pressure correlates with low cardiac output. Systolic ventricular function is not impaired.