Consciousness disorders occur in most patients admitted into intensive care units. One of their rare causes may be hyperammonemia. A 28‑year‑old male was admitted to the intensive care unit due to a consciousness disorder of an unspecified origin. Alcohol, drug or other psychoactive medicament poisoning was excluded. In childhood he was diagnosed with hyperammonemia type 2 – for many years he has not been following the recommended therapy. On admission he was drowsy, without any logic contact, sometimes excited. Cerebral edema was excluded after performing a CT scan. In laboratory blood tests it was alleged that only hypoglycemia (40 mg/dl), high ammonia concentration (140 mcg/dl) and hyperbilirubinemia (3.52 mg/dl) were present. The sedation, continuous infusion of glucose, an appropriate diet and intravenous medicines reducing ammonia serum concentration were applied. Consciousness disorders were still observed, therefore it was decided to start continuous venovenous hemodiafiltration – this caused that the general state of the patient to get better, and the patient was discharged home. Hyperammonemia should be considered as one of the causes of consciousness disorders. Quick and correct diagnosis and treatment allows doctors to avoid the damage to the central nervous system and the patient’s death.