Prior studies have demonstrated successful irritability treatment using dopaminergic antagonists in autistic patients. The purpose of this pilot study was to assess the effect of dextromethorphan/quinidine (DM/Q) in autistic adults (18-60 years of age). This was a randomized, blinded, crossover, study of 14 patients randomized to DM/Q or a placebo for 8 weeks, washed out for 4 weeks, then crossed over to the opposite treatment. There were no serious adverse events. Subjects were significantly lower on the Aberrant Behavioral Checklist for Irritability (ABC-IR) (F[subscript 1,10] = 7.42; p = 0.021). Improvements in aggression and Clinical Global Impression were also seen. The findings suggest that DM/Q is well-tolerated and associated with improvements in irritability and aggression in adults with autism.