Levels of serum cholesterol, triglycerides, lipoprotein pattern and the insolubilization of serum low density lipoproteins (LDL, betalipoprotein) by a factor present in arterial intima--media extracts was investigated in 55 patients with acute coronary heart disease and 50 healthy controls. In blood samples obtained 24 h after the episode, the serum betalipoproteins from male normotensive patients showed a high tendency to precipitate when incubated with the intima-media extracts, nearly twice the value measured in the control group. This affinity returned almost to control level after 21 days in the hospital. Hypertensive patients showed a serum cholesterol higher than the controls but almost no difference in LDL-arterial factor interaction. The composition of isolated LDL was studied in 7 patients and 8 controls and it was found that the total cholesterol/protein plus phospholipid ratio has a positive exponential correlation with the lipoprotein-arterial factor affinity. The results of LDL analyses suggest that a change in their composition, frequent in normotensive patients, can lead to an increased affinity of this lipoprotein for arterial intimamedia components. It is proposed that this process could be an additional risk factor contributing to the deposition of LDL in the arterial wall.