Both genome-wide heterozygosity and heterozygosity at major histocompatibility complex (MHC) genes are often associated with higher fitness. Recent theoretical work indicates that sexual ornaments may reveal information about individual heterozygosity, and that preference for such ornaments may benefit females via the increased heterozygosity of their progeny. Here, we used path analysis to investigate the direct and indirect (via body size used as an index of condition) effects of heterozygosity at six microsatellite loci and the MHC class II DAB gene on the size of a sexual ornament, the crest, in the crested newt Triturus cristatus. We found that microsatellite heterozygosity, but not MHC heterozygosity, significantly predicted male body size, and that male body size significantly predicted crest height. However, there was no direct effect of MHC or microsatellite heterozygosity on crest height. Furthermore, microsatellite heterozygosity significantly increased with age, indicating that it had a positive effect on survival. Overall, our results are consistent with the hypothesis that heterozygosity determines condition, and that variation in condition is expressed as variation in sexual ornamentation.