Plasma `cortisol' values are reported in 36 patients during or following right ventricular failure, and 12 during or following left ventricular failure. In patients with right ventricular failure the normal circadian rhythm was abolished with elevation of midnight values. Disturbance of rhythm was less marked in left ventricular failure, although midnight values were usually raised. Dexamethasone suppression of plasma `cortisol' levels was abolished or reduced in right ventricular failure. These results confirm our earlier findings and further support our contention that the disturbance of circadian rhythm is not due to a simple stress mechanism. The results are of importance in the differential diagnosis of Cushing's syndrome based on plasma `cortisol' values, if there is concomitant right ventricular failure.