The concentrations of serum total and HDL cholesterol and triglycerides were determined in 57 patients during the course of AMI. In seven days the concentration of serum cholesterol decreased by 24% and that of HDL cholesterol by 31%. The mean HDL/total cholesterol ratio decreased significantly (p less than 0.01) from 0.163 to 0.145. The magnitude of the change in both HDL and total cholesterol showed a positive correlation with infarction size. The concentration of triglycerides decreased in seven days on the average by 31% but there was great individual variation, which was not dependent on infarction size. Four months after infarction both HDL and total cholesterol as well as triglyceride concentrations had returned to the initial levels. There was a significant negative correlation between the concentrations of HDL cholesterol and triglycerides on admission (r = -0.66) and after four months (r = -0.53) but no correlation after seven days. The results indicate that the determination of serum lipids, including HDL cholesterol, in patients with AMI can, and should be performed on admission to hospital and not at the time of discharge, in order to get reliable estimates of these cardiovascular risk factors.