We investigate the contribution of overdominance to the maintenance of polymorphism in the human genome during the recent evolution of our species. Using the HapMap genotypic information, we have detected that the Gene Ontology term "olfactory receptor activity" is a molecular function overrepresented in genes that have SNPs (Single Nucleotide Polymorphisms) showing higher than expected number of heterozygotes in the HapMap populations. Our results suggest that the diversity of a subset of human olfactory receptors (ORs) may have been maintained by balancing selection, in the form of overdominance. This observation may suggest that the loss of OR genes during the evolution of the human lineage may have been accompanied by an increased capability to discriminate odorants with closely similar structures.