A 17-year-old girl with Rett syndrome, who was taking no other medications, was treated with L-carnitine (50 mg/kg/day). Within 2 months of initiation of treatment, she became much more alert, developed good eye contact, started reaching for objects with both hands, and answered simple questions with one or two words. Lcarnitine was discontinued and within 1 week she lapsed into her pretreatment condition of lethargy with no interest in her environment, not reaching for objects, poor eye contact, and not speaking. One week after L-carnitine was resumed, she again became alert, started reaching for objects, and saying one or two words. Her serum carnitine levels (free and total) were within normal limits before and after L-carnitine treatment, but were higher while she was taking L-carnitine. Her serum ammonia was within normal limits prior to starting L-carnitine. L-carnitine appears to be an effective treatment for this girl with advanced Rett syndrome.