To report an unusual case of idiopathic zygotic polypronuclei. Case report. Ma jor urban infertility referral center. A 34-year-old woman with unexplained in fertility. The patient underwent two cycles of controlled hyperstimulation and o ocyte retrieval followed by in vitro insemination and intracytoplasmic sperm inj ection (ICSI). Pregnancy and delivery of a normal infant following transfer of a single preembryo from an ICSI cycle in which only one zygote showed a normal pr onuclear number (2PN) and 12 zygotes appeared polypronucleated (< 3PN). On the f irst IVF cycle, 23 oocytes were retrieved and inseminated with 240×103 motile s perm/mL, after which two zygotes showed a normal pronuclear number and 20 zygote s appeared polyploid with three to seven pronuclei. Transfer of two poorquality day-3 preembryos following assisted hatching did not achieve pregnancy. On the subsequent ICSI cycle, 33 oocytes were retrieved, and 17 mature oocytes were sub jected to ICSI, after which only one zygote showed 2PN and 12 zygotes appeared polyploid with three to eight pronuclei. Th e normally fertilized zygote developed into a poor-quality, day-3 embryo and w as subjected to assisted hatching. Transfer of this preembryo resulted in an une ventful pregnancy and birth of a normal infant. Mechanisms other than polyspermi a may result in polypronuclear development in some patients. To report an unusual case of idiopathic zygotic polypronuclei. Case report. Ma jor urban infertility referral center. A 34-year-old woman with unexplained in fertility. The patient underwent two cycles of controlled hyperstimulation and o ocyte retrieval followed by in vitro insemination and Intracytoplasmic sperm inj ection (ICSI). Pregnancy and delivery of a normal infant following transfer of a single preembryo from an ICSI cycle in which only one zygote showed a normal pr nuclear number (2PN) and 12 zygotes have been polypronucleated (<3PN). On the f irst IVF cycle, 23 oocytes were retrieved and inseminated with 240 × 10 3 motile s perm / mL, after which two zygotes showed a normal pronuclear number and 20 zygote s, which had a polyploid with three to seven pronuclei. Transfer of two poorquality day-3 preembryos following assisted hatching did not achieve pregnancy. On the subsequent ICSI cycle, 33 oocytes were retrieved, and 17 mature oocytes were sub jected to ICSI, after which only one zygote showed 2PN and 12 zygotes had polyploid with three to eight pronuclei. Th e normally fertilized zygote developed into a poor-quality, day-3 embryo and w as subjected to assisted hatching. Transfer of this preembryo resulted in an une ventful pregnancy and birth of a normal infant. Mechanisms other than polyspermi a may result in polypronuclear development in some patients.