A 3-year-old girl presented to the emergency department with seizures, low-grade fever and vomiting. She had tachycardia and a slow capillary refill. Blood pressure could not be measured. Because of suspected sepsis and/or meningo-encephalitis, broad spectrum antibiotics and antiviral medication were given together, along with volume expansion and anticonvulsive therapy. A few hours later, after a second seizure, the blood pressure was extremely high (156/116 mm Hg). The girl was treated with anticonvulsants and intravenous antihypertensive agents. MRI of the brain showed signs of posterior reversible encephalopathy syndrome. Cultures of blood and cerebrospinal fluid remained sterile. Further investigation into the cause of the malignant hypertension revealed hypokalemia, metabolic alkalosis and extremely high plasma renin activity, caused by a rare renal abnormality: bilateral renal segmental hypoplasia or Ask-Upmark kidneys.