Williams syndrome is a relatively rare genetic disorder caused by the hemizygous microdeletion of a region in chromosome 7q11.23. Individuals with Williams syndrome typically present with a highly social, overfriendly, and empathic personality. Comorbid medical and neuropsychiatric disorders are common. Reports of effective pharmacological treatment of associated neuropsychiatric disorders are limited. The authors describe the successful treatment of interfering anger, aggression, and hair-pulling with N-acetylcysteine in a 19-year-old woman with Williams syndrome. The neuropsychiatric symptoms emerged 1 week following an upper gastrointestinal endoscopy, for which fentanyl, midazolam, and propofol were used as anesthetics. The patient’s treatment course and hypothesized mechanisms underlying the clinical presentation and symptom resolution are described.