BACKGROUND: Normal fertilization is usually considered to have occurred when two pronculei (2PN) and two polar bodies are observed. Exceptions are the single pronucleated zygote resulting from asynchronous pronuclei. CASE: A 29-year-old woman entered a program of intracytoplasmic sperm injection and embryo transfer because of her husband's oligoasthenoteratozoospermia. Two cleavage-stage embryos (four blastomeres, grade 1 and 2) were obtained from one fertilized oocyte containing distinct 2PN and the other a single pronucleus (1PN). At 15 weeks' gestation the patient developed severe preeclampsia requiring termination of the pregnancy. Histopathologic examination and DNA ploidy by image analysis were consistent with a twin pregnancy combining a complete hydatidiform mole and normal pregnancy. CONCLUSION: We hypothesize that this 1PN was at the origin of the hydatidiform mole. This case highlights the danger of transferring an embryo having 1PN.