It is likely that there is an association between the fitness of mitochondria and their ability to support normal cellular function. Oocytes are greatly enriched in the number of mitochondria as they support essential developmental processes such as oocyte maturation and embryonic development, while their replication is deferred until gastrulation. The mitochondrial DNA (mtDNA) copy number in 87 human oocytes from 29 patients was evaluated after DNA extraction and real-time quantitative polymerase chain reaction (PCR). The average mtDNA copy number was 795,000 (+/- 243,000) in metaphase II oocytes. mtDNA content varied considerably between oocytes, even within the same patient. No relationship was found between mtDNA copy number and maternal age. The findings suggest that mtDNA replication is fully accomplished by the germinal vesicle stage in the fully developed oocyte.