ABSTRACT: INTRODUCTION: The discovery of a mature teratoma (dermoid cyst) of the ovary during ovarian stimulation is not a rare event. Conversely, we could not find any reported cases of immature teratoma in such a situation. Clinical and ultrasound arguments for this immature form are scarcely or poorly evaluated. Case Presentation: We describe the case of a 31-year-old Caucasian woman with primary infertility, who developed an immature teratoma during an in vitro fertilization ovarian stimulation cycle. CONCLUSIONS: Ultrasound signs of an atypical cyst during ovarian stimulation allowed us to adopt a careful medical attitude and to adapt the required surgical oncological treatment.