OBJECTIVE--To compare serum concentrations of total cholesterol, low density lipoprotein (LDL) cholesterol, high density lipoprotein (HDL) cholesterol, and triglycerides four weeks after acute myocardial infarction with baseline levels measured within 24 hours after onset of symptoms. DESIGN--A prospective study including 141 patients with acute myocardial infarction who were admitted to the coronary care unit at a general hospital. MEASUREMENTS--Fasting serum concentrations of total cholesterol, LDL cholesterol, HDL cholesterol, and triglycerides. MAIN RESULTS--In patients receiving thrombolytic therapy, no significant differences were found in serum lipids four weeks after admission compared to values estimated within 24 hours from onset of symptoms. In patients not receiving thrombolytic therapy, total cholesterol and low density lipoprotein cholesterol showed a minor increase four weeks after admission compared to values obtained within 24 hours after onset of symptoms. High density lipoprotein cholesterol and triglycerides remained unchanged. CONCLUSIONS--In patients with acute myocardial infarction receiving thrombolytic therapy, serum lipids measured four weeks after onset of infarction are reasonably valid estimates of baseline lipid levels and may be used to decide about lipid lowering interventions. This information can be a basis for actions against hyperlipidaemia early after hospital discharge when the patient is highly motivated to change lifestyles and is still in close contact with a cardiologist or other physician.