Natural selection shapes many human genes, including some related to complex diseases. Understanding how selection affects genes, especially pleiotropic ones, may be important in evaluating disease associations and the role played by environmental variation. This may be of particular interest for genes with antagonistic roles that cause divergent patterns of selection. The lectin-like low-density lipoprotein 1 receptor, encoded by OLR1, is exemplary. It has antagonistic functions in the cardiovascular and immune systems because the same protein domain binds oxidized low-density lipoprotein and bacterial cell wall proteins, the former contributing to atherosclerosis and the latter presumably protecting from infection. We studied patterns of selection in this gene, in humans and nonhuman primates, to determine whether variable selection can lead to conflicting results in cardiovascular disease association studies.We analyzed sequences from 11 nonhuman primate species, as well as single-nucleotide polymorphisms and sequence data from multiple human populations. Results indicate that the derived allele is favored across primate lineages (probably because of recent positive selection). However, both the derived and ancestral alleles were maintained in human populations, especially European ones (possibly because of balancing selection derived from dual roles of LOX-1). Balancing selection likely reflects response to diverse environmental pressures among humans.These data indicate that differential selection patterns, within and between species, in OLR1 render association studies difficult to replicate even if the gene is etiologically connected to cardiovascular disease. Selection analyses can identify genes exhibiting gene-environment interactions critical for unraveling disease association.