We assessed the reliability of early (first day) plasma lipid measurements in patients admitted with acute myocardial infarction or unstable angina pectoris. In 55 such patients, plasma levels of cholesterol and triglycerides measured within the first 24 hours after admission were significantly lower than the corresponding values at 6 weeks, which means that even very early in-hospital lipid measurements could lead to potentially serious underestimation of the lipid risk in these patients ad defined by the current criteria.