The serum levels of Lp(a) lipoprotein (Lp(a)) were determined preoperatively in 601 patients with coronary artery disease, undergoing bypass operations. Compared with a reference group of 99 apparently healthy individuals, the Lp(a) levels were higher in the patient group (7.7 mg dl-1 vs. 5.1 mg dl-1, p = 0.012). In the patient group, there was a weak, but significant negative correlation between the Lp(a) levels and age (r = -0.10, p = 0.017), and in both groups the women had higher Lp(a) levels than the men. In the patients we found no significant correlations between Lp(a) and other serum lipids or lipoproteins, nor between Lp(a) and variables in the fibrinolytic system. We investigated the long-term effects of supplementation with n-3 polyunsaturated fatty acids (n-3 PUFAs) on the Lp(a) concentrations. Postoperatively, in a randomized fashion, 280 of the patients received 4 g of an n-3 PUFA concentrate (containing > 85% of long-chain n-3 PUFAs) per day, whereas 269 patients comprised the control group. The fatty acids in serum phospholipids were monitored, and a significant increase in the phospholipid n-3 fatty acids was noted in the n-3 PUFA group, as opposed to the virtually unchanged amounts in the control group. The Lp(a) levels were determined again after 6 months, and, compared with the control group, n-3 PUFA supplementation had no overall effect on the serum Lp(a) levels.