Dextromethorphan is the d-isomer of levorphenol, and an ingredient in antitussive preparations. A 10 year-old male diagnosed with Autistic Disorder, Pervasive Developmental Disorder, and Generalized Anxiety Disorder was administered this medication initially to treat a medical condition. This became a quasi-experimental ABAB design (A = baseline, B = treatment), with improvements during treatment phases shown in tracked behavioral data and anecdotal reports. Several candidate mechanisms to explain the improvements are offered, including glutamate receptor antagonism. While dextromethorphan is not commonly administered for the treatment of behavioral challenges in this or any population, the results suggest the need for larger-scale, adequately controlled, and methodologically rigorous studies of the potential clinical effects of dextromethorphan.