Ornithine transcarbamylase deficiency (OTCD) is an X-linked inherited disease and the most common inborn error in urea synthesis in human patients. In adult heterozygous patients, OTCD can be responsible for life-threatening hyperammonemic coma. We report the case of a 32-year-old woman admitted to our hospital with seizures after a recent high protein load. Her parents related a history of recurrent episodes of vomiting, meat refusal, lethargy, and convulsions since childhood, and measurement of plasma ammonemia levels was the key to early diagnosis of OTCD. We report the pathophysiologic characteristics, clinical features, clinical course, and differential diagnosis of OTCD and discuss the therapeutic options, including continuous venovenous hemodiafiltration and pharmacologic therapy for reduction of plasma ammonemia levels. A diagnosis of OTCD should be considered in adult nonhepatic patients with hyperammonemic coma, particularly if they have a history of protein avoidance and neurologic symptoms. Early recognition and appropriate treatment are critical to avoid severe brain damage and death.