The associations previously found between lipoprotein(a) (Lp(a)) levels and atherosclerotic disorders, diabetes, rheumatoid arthritis and renal diseases suggest that Lp(a) may be involved in autoimmune reactions. The relation found between Lp(a) levels and the HLA class II genotype in males with early coronary artery disease (CAD) further support that assumption. It was suggested that an autoimmune process, perhaps triggered by a concomitant intracellular infection may occur especially in patients with inherited high Lp(a) levels in combination with certain inherited HLA class II genotypes. In this study a Chlamydia pneumoniae IgG titer > or = 32 was significantly more common (P = 0.036) in CAD patients than in matched controls. This is in agreement with previous reports by other investigators. In addition, an IgG titer > or = 256 in combination with an Lp(a) level > or = 120 mg/l was found to occur significantly more often (P = 0.011) in male patients than in male controls. Certain HLA class II DR genotypes in combination with high Lp(a) levels and C. pneumoniae titers occurred more frequently in both male and female patients than in controls. Some combinations were very common in male patients, and the difference in comparison with male controls was highly significant.