During intracytoplasmic sperm injection (ICSI) the whole sperm, including head, midpiece and tail, is injected into the middle area of the oocyte. To find out what happens to the sperm mitochondria after ICSI, we checked the first six children born after ICSI treatment for occurrence of paternal mitochondrial DNA (mtDNA). The difference between maternal and paternal mtDNA in the investigated couples in our study was confined to single-base pair substitutions and we had to rely on restriction enzyme cleavage to differentiate between the mitochondrial genomes of the parents. With this kind of assay we were able to reach a sensitivity of about 0.2% for the paternal mtDNA. However, as uneven partition between tissues of heteroplasmic mtDNA is expected to occur, it would not be unlikely that an enrichment to 0.2% would occur in a given tissue if paternal mtDNA was transmitted by the ICSI procedure. We did not detect this level in the blood in any of the six children.