A partial N-methyl-D-aspartate agonist, D-cycloserine, enhances fear extinction when given before or shortly after exposure to feared stimuli in animals. In this pilot double-blind placebo-controlled trial (trial number: ISRCTN70977225), 27 youth with obsessive-compulsive disorder were randomised to either 50 mg D-cycloserine or placebo administered immediately after each of ten cognitive-behavioural therapy (CBT) sessions, primarily consisting of exposure and ritual prevention. Both groups improved significantly and maintained their gains at 1-year follow-up, with no significant advantage of D-cycloserine over placebo at any time point. The effects of CBT may not be augmented or accelerated when D-cycloserine is administered after sessions.