Genetic diversity is one of the three forms of biodiversity recognized by the World Conservation Union (IUCN) as deserving conservation. The need to conserve genetic diversity within populations is based on two arguments: the necessity of genetic diversity for evolution to occur, and the expected relationship between heterozygosity and population fitness. Because loss of genetic diversity is related to inbreeding, and inbreeding reduces reproductive fitness, a correlation is expected between heterozygosity and population fitness. Long-term effective population size, which determines rates of inbreeding, should also be correlated with fitness. However, other theoretical considerations and empirical observations suggest that the correlation between fitness and heterozygosity may be weak or nonexistent. We used all the data sets we could locate (34) to perform a meta-analysis and resolve the issue. Data sets were included in the study, provided that fitness, or a component of fitness, was measured for three or more populations along with heterozygosity, heritability, and/or population size. The mean weighted correlation between measures of genetic diversity, at the population level, and population fitness was 0.4323. The correlation was highly significant and explained 19% of the variation in fitness. Our study strengthens concerns that the loss of heterozygosity has a deleterious effect on population fitness and supports the IUCN designation of genetic diversity as worthy of conservation.