We have evaluated overall and cardiac sympathetic activity in 47 patients undergoing coronary angiography, 27 with stable angina of at least 3 months duration, and 20 with unstable ischaemic symptoms within this period. Cardiac and overall sympathetic activity were assessed using radiotracer noradrenaline kinetic techniques to measure cardiac and total noradrenaline spillover to plasma. Overall sympathetic activity (whole body noradrenaline spillover) was similar in the two groups, whereas cardiac sympathetic activity (cardiac noradrenaline spillover) was strikingly increased in the patients with unstable ischaemic symptoms (102 +/- 23 pmol.min-1 vs 34 +/- 4 pmol.min-1, P < 0.001), as was the cardiac to whole body noradrenaline spillover ratio (0.043 +/- 0.008 vs 0.021 +/- 0.005, P < 0.01). Coronary sinus bloodflow (50 +/- 4 ml.min-1 vs 38 +/- 4 ml.min-1, P < 0.05) and coronary sinus noradrenaline concentration (2.60 +/- 0.38 nmol.l-1 vs 1.41 +/- 0.17 nmol.l-1, P < 0.01) were also increased in the patients with unstable ischemic syndromes. Left ventricular ejection fraction was similar in the two groups (63 +/- 2% vs 62 +/- 2%). Patients with unstable ischaemic symptoms within the previous three months have increased cardiac sympathetic nervous activity compared to patients with stable angina. This may in part explain why patients with unstable ischaemic syndromes are at increased risk of sudden cardiac death.