AbstractBackgroundThe amino acid l‐citrulline is used as a therapeutic agent for urea cycle disorders (UCD) including ornithine transcarbamylase deficiency (OTCD), carbamoyl phosphate synthetase I deficiency (CPSD), and N‐acetylglutamate synthase deficiency. There are few reports, however, on the use of l‐citrulline in Japan and little consensus regarding the effects of l‐citrulline.MethodsWe conducted a questionnaire survey of patients undergoing l‐citrulline treatment for a UCD to evaluate the current status of this therapy. The survey included patient background, details of l‐citrulline treatment, clinical examination data, treatment, frequency of vomiting, and liver transplantation.ResultsWe retrospectively investigated 43 questionnaire respondents (OTCD, n = 33; CPSD, n = 10). The weight of male OTCD patients improved by +0.79 SD, and the ammonia level decreased by a mean of 44.3 μmol/L in all patients. The protein intake of all patients and of male OTCD patients increased by 0.14 g/kg/day and 0.17 g/kg/day, respectively.Conclusionsl‐Citrulline effectively reduced ammonia level, increased protein intake, and improved weight gain in UCD patients. l‐Citrulline should be considered a standard therapy in OTCD and CPSD patients.