Context: Spontaneous ovarian hyperstimulation syndrome (SOHSS) is exceptionally rare. Associations with hypothyreoidism, pituary adenoma or mutation of the FSH receptor were described. Polycystic ovarian syndrome (PCOS) is a risk factor for iatrogenic OHSS, however it has only been reported once in association with SOHSS. In rats, diabetic hyperglycemia increases the risk for systemic capillary leak. Objective/Method: Case Report. Participant: 32 y.o. woman with diabetes mellitus (DM) type 1, nephropathy grade 5, arterial hypertonia (AH), chronic anemia and normal TSH levels. The gynecological history revealed that despite of acne, hirsutism, and oligo-/amenorrhea, along with elevated DHEA-S (4.96-5.54 mg/l) and total testosterone (TT) (0.72 ng/ml) levels, she was never diagnosed with and treated for PCOS. Four years after the initial hormonal assay, she repeated the investigations: DHEA-S levels remained elevated, but TT fell to 0.26 ng/ml. In the same year, the menstruations became more regular. Intervention: Two months after last period, she was admitted to the medical ward of an external hospital due to DM exacerbation. Unexpectedly, the serum hCG was 111522 U/ml. The transvaginal ultrasound scan confirmed a viable intrauterine pregnancy (CRL 7.5 mm, corresponding to 6+5 gestational weeks [gw]). Furthermore, a septated cyst (58x34 mm) in the left ovary in absence of free abdominal fluid was described. Medical treatment was then continued in the department of internal medicine and endocrinology at a university hospital. After 9 days, the patient was discharged from there with the additional diagnosis of hyperemesis gravidarum. 12 days later (9+6 gw) she was admitted to our department with the diagnosis of an early miscarriage. Therapeutic D&C after cervical priming with misoprostol was performed. Postoperatively, the patient developed abdominal pain, nausea, hypoalbuminemia, hyperglycemia and exacerbation of AH, and was admitted to the intermediate care unit of our hospital. The postoperative ultrasound revealed massive ascites, a multilocular left ovary (110 x 60 mm) and slight enlarged right ovary (41x30 mm). Result: SOHSS and (retrospectively) PCOS were diagnosed. The patient was discharged after 10 days, following regression of symptoms. Conclusion: We report the first case of SOHSS diagnosed after spontaneous miscarriage. The predisposing role of previously not recognized PCOS and/or diabetic hyperglycemia can be supposed.