Although extra-pair fertilizations (EPF) are common in socially monogamous systems, the benefits to females remain elusive. One potential benefit that recently has begun to receive empirical attention is increased offspring heterozygosity. We tested the heterozygosity hypothesis in the black-throated blue warbler Dendroica caerulescens using a panel of five microsatellite loci. We did not find any evidence that male heterozygosity influenced female extra-pair mating patterns, that females choose genetically dissimilar extra-pair mates, nor that extra-pair offspring were more heterozygous than within-pair offspring. However, simple Monte Carlo simulations indicated that the number of microsatellite loci used in this study, and most other recent studies, would detect an effect of heterozygosity only if that effect is pronounced. Thus, we were unable to demonstrate any effect of offspring heterozygosity in our study species, but a more subtle effect remains possible. Researchers wishing to test this hypothesis should use a large number of genetic markers and interpret negative results cautiously.