Objective: The authors assessed the effects Of D-cycloserine on the core symptom of social impairment in subjects with autism. Method: Following a 2-week, single-blind placebo lead-in phase, drug-free subjects with autistic disorder were administered three different doses Of D-cycloserine during each of three 2-week periods. Measures used for subject ratings included the Clinical Global Impression (CGI) scale and Aberrant Behavior Checklist. Results: Significant improvement was found on the CGI and social withdrawal subscale of the Aberrant Behavior Checklist. D-Cycloserine was well tolerated at most of the doses used in this study. Conclusions: In this pilot study, D-Cycloserine treatment resulted in significant improvement in social withdrawal. Further controlled studies Of D-cycloserine in autism appear warranted.