There is emerging evidence that early onset obsessive-compulsive disorder (OCD) may be a phenomenologically distinct subtype of the disorder. Previous research has shown that individuals who report an early onset display greater severity and persistence of symptoms, and they may be less responsive to treatment. To date, this question has been investigated solely in adult samples. The present study represents the first investigation into the effect of age at onset of OCD on clinical characteristics and response to treatment in a paediatric sample.