Primary bacterial peritonitis is a rare complication of idiopathic nephrotic syndrome (INS) in children, found in 1.5-3.7% cases. The 10-year-old girl was admitted with INS relapse: generalized edema, proteinuria 630 mg/kg/24 h, hypoalbuminemia 1.8 g/dL, hypogammaglobulinemia 74.0 mg/dL (n: 618-1537 mg/dL), GFR 71.6 mL/min/1.73 m2. She was treated with prednisone 60 mg/24 h. On 5th day severe pain, fever, CRP (15.5 mg/dL) and leukocytosis (19.5 tys/mm3) rise occurred. On 6th day due to suspicion of peritonitis, laparotomy was performed and 400 mL of suppurative exudate was evacuated (Streptococcus pneumoniae was cultured). Postoperative course was complicated with acute kidney injury (GFR 47.7 mL/min/ 1.73 m2), lung edema, arterial hypertension, and separation of the layers of a surgical wound. The patient was treated with: imipenem (9 days), vancomycine i.v. (4 days)/p.o. (11 days) (Clostridium difficile toxin present in stool), fluconazole (14 days), 20% albumins, furosemide, labetalole, cyclosporine A (started on 56th day after the operation due to secondary steroid-resistance of INS). The remission was achieved after 7 days of cyclosporine A treatment. Authors suggest that children with nephrotic syndrome belong to high-risk group of invasive pneumococcal disease, therefore they require careful implementation of mandatory immunization schedule. Peritonitis is a rare and still dangerous infectious complication of nephrotic syndrome in children.