Of nine patients with pericardial effusion due to various causes, four developed cardiac tamponade. Electrical alternans was present in all four, being total in three and ventricular in one. The alternans corresponded very well with the clinical diagnosis of cardiac tamponade and the radiological signs of a large pericardial effusion. In two patients alternans was present even with heart rates below 100 per minute. Apart from the exact (1 : 1) type of electrical alternans, three new types are described, a 2 : 1, 3 : 1, and a varying type. It is concluded that (a) electrical alternans associated with pericardial effusion is strongly suggestive of impending or established cardiac tamponade, and (b) electrical alternans is produced when the heart is oscillating within the pericardial sac distended by fluid with a frequency equal to one-half (exact alternans), one-third (2 : 1 alternans), and one-quarter (3 : 1 alternans) of the heart rate. The aetiology and mechanism of electrical alternans are discussed.