BACKGROUND: Meta-analyses support the efficacy of cognitive behavioural therapy (CBT) for obsessive-compulsive disorder (OCD) in Western cultures. However, there are no adequately powered multicentre studies in China. This study aimed to compare the effectiveness of treatment with CBT combined with medication and medication alone in OCD patients in China. METHODS: OCD patients (N = 167) were recruited from outpatient clinics at three large tertiary psychiatric hospitals and one general hospital in China. Participants were randomly allocated to receive either CBT combined with medication (n = 92) or medication alone (n = 75) for a 24-week treatment period. Participants' symptoms and social functioning were assessed using the Yale-Brown Obsessive-Compulsive Scale (Y-BOCS), Hamilton Anxiety Rating Scale (HAM-A), Global Assessment of Functioning (GAF) and Clinical Global Impression Scale for Severity (CGI-S) at 0, 4, 8, 12 and 24 weeks, and the effectiveness of the two treatments compared using linear mixed-effects models. RESULTS: At 24 weeks, both groups showed large within-group effects in all measures. Significantly more patients receiving combined therapy than medication alone had a decrease in symptom severity of at least 35% (based on Y-BOCS total score). The CGI-S and GAF scores decreased in both groups, and significant differences were found between the groups. LIMITATIONS: Study limitations included lack of consideration of medication types and dosages, and the absence of a CBT-only arm. CONCLUSIONS: CBT combined with medication may be effective in alleviating symptoms and social functioning impairment associated with OCD, and is more effective than medication alone in China, particularly for the treatment of compulsive behaviours.