Background, rationale, and methods . Lesch-Nyhan disease is a rare, X-linked disorder due to hypoxanthine phosphoribosyltransferase deficiency. To evaluate reported benefit on mood and behavior obtained by the administration of S-adenosyl-l-methionine in this condition, we developed 2 quantitative evaluation tools, and used them to assess the effects of the drug in our population: the weekly questionnaire and the resistance to self-injurious behavior test. We performed an open-label, dose-escalation trial of the drug on 14 patients. Results. Four patients tolerated the drug and reported beneficial effects. The majority experienced worsened behavior. The 2 assessment tools demonstrated effectiveness in quantitatively evaluating the self-injurious behavior.