Current medical textbooks do not give clear advice on the optimal time to measure plasma lipids following myocardial infarction. Many physicians still defer the measurement for three months. We studied the plasma lipids in 132 patients with myocardial infarction. Measurements were made on admission and 24, 48 and 72 hours later and 103 patients had their plasma lipids measured at three months. Forty-three patients had hypercholesterolaemia (cholesterol greater than 7 mmol/1) at admission. Total cholesterol and low density lipoprotein fell appreciably in the first three days after myocardial infarction. The former returned almost to its admission level by three months, whilst the latter did not. Triglycerides showed no important changes, whilst high density lipoprotein levels had significantly increased at three months. Total cholesterol levels at three months were different from the 24, 48 and 72 hour values. Our findings suggest that plasma lipids should be measured on admission after myocardial infarction. Deferring the measurement by one or two days will give results which may not reflect pre-infarction levels.