With the identification of a patient with mutated mitochondrial DNA (mtDNA) of paternal origin, it has been unequivocally proven that not only does paternal mtDNA survive in the zygote, but it can also contribute substantially to the mtDNA pool of adult, human skeletal muscle. The questions are: how often does paternal mtDNA inheritance occur and what mechanisms are involved? In this paper, we will review current knowledge on the fate of sperm mitochondria after fertilization and discuss the impact paternal inheritance may have on our understanding of mitochondrial biology. (C) 2003 Elsevier Inc. All rights reserved.