Ascites with elevated serum CA-125 but without suspicious adnexal masses is a diagnostic challenge. The differential diagnosis of patients aged 20–50 years old presenting with ascites should account for rare etiologies, such as the hypereosinophilic syndrome. A 37-year-old patient was referred to the Gynecologic Oncology Outpatient Clinic of the University Hospital in Krakow due to rapidly progressing ascites with serum CA-125 above the normal limit. Thorough imaging studies showed no evidence of malignant ascites, whilst hypereosinophilia was detected in laboratory tests. After ruling out other causes of primary and reactive hypereosinophilia, the diagnosis of the hypereosinophilic syndrome was made. After the administration of oral corticosteroid therapy, complete resolution of symptoms (including ascites) was achieved, and CA-125 level returned to normal. Currently, the patient is asymptomatic and is followed up at the Autoimmune Diseases Outpatient Clinic.