Little is known about racial differences in lipoprotein [a] (Lp[a]) concentrations and apolipoprotein[a] (apo[a]) phenotypes. Lp[a] protein concentrations were determined by a double monoclonal antibody enzyme-linked immunosorbent assay method in 4165 Caucasian and African American men and women from four US communities. Apo[a] phenotypes were determined by polyacrylamide gel electrophoresis and immunoblotting on a random subset of these participants (n=690). The distribution of Lp[a] protein levels in Caucasians was highly skewed (mean, 6.9 mg/dL; median, 3.7 mg/dL). In contrast, the distribution in African Americans was less skewed (mean, 13.0 mg/dL; median, 11.6 mg/dL), and Lp[a] protein levels were approximately double those in Caucasians within most apo[a] phenotypes. The previously described inverse relationship between apo[a] size and Lp[a] concentration was generally confirmed in Caucasians, but the B phenotype had lower Lp[a] levels than the SI or S2 phenotype. In African Americans, both the B and SI phenotypes had lower Lp[a] levels than the S2 phenotype. The frequencies of the apo[a] phenotypes in African Americans differed from those in Caucasians (p<.001) and also differed from the frequencies reported in a Sudanese population (p<.002). African Americans had a lower frequency of the S2 phenotype than Caucasians (8% vs 18%;p<.01) and a higher frequency of S3 (36% vs 25%;p<.01). As compared with the data reported in Sudanese, African Americans also had a higher frequency of the S3 phenotype (36% vs 14%; P-c.OOl) and a lower frequency of S4 (29% vs 44%;p<.01). The apo[a] polymorphism explained 35% of the variability in Lp[a] concentrations in Caucasians and 27% of the variability in African Americans. Though frequencies of the apo[a] phenotypes differ between Caucasians and African Americans, they do not explain the differences in Lp[a] levels between the two racial groups.