The clinical significance of serial central venous oxygen saturation (CVSo ) determinations was investigated in 34 patients with acute myocardial infarction. The mean initial CVSo2 of 73% fell to a low of 68% (P<0.05) on the third postinfarction day, and rose slightly thereafter. The average values of the lowest recorded CVSo2 in 9 patients with an uncomplicated course, 10 patients with a moderately complicated course, and 15 patients with a severely complicated clinical course were 65%, 65%, and 54%, respectively. A fall in CVSo2 to less than 55% preceded the development of hypotension in three patients and was concurrent with hypotension in two. Serial CVSo2 determinations are useful in the early detection of cardiac decompensation in patients with acute myocardial infarction.