Nutritional management of patients who have urea cycle disorders is one of the most challenging tasks in clinical nutrition. The degree to which protein intake should be restricted in urea cycle disorders requires complex calculations which depend on many variables such as specific enzyme defect, age-related growth rate, current health status, level of physical activity, amount of free amino acids administered, energy intake, residual urea cycle function, family lifestyle, use of nitrogen-scavenging medications, and the patient's eating behaviors. This paper presents two case histories and a series of recommendations outlining the nutrition management of urea cycle disorders. It also identifies difficulties that arise in the course of treatment, and suggests practical solutions for overcoming them.