It is often suggested that early onset of disorders leads to higher severity and greater treatment refractoriness. Previous research has investigated whether there are clinical and demographic differences between groups of individuals who have experienced onset of obsessive-compulsive disorder (OCD) at an early or later age. Results suggest that individuals who report an early onset (EO) of the disorder report greater severity and persistence of symptoms. However, few studies have investigated whether there are differences in treatment response. The present study represents a preliminary investigation in the setting of a specialist OCD clinic. Analysis was undertaken using clinical records to investigate whether there were clinical differences and in response to cognitive-behavioural treatment (CBT) between individuals who developed OCD at an early (n=22) or late (n=23) age. No differences in treatment response between the groups were found. However, the EO group were rated as being more severe across a range of measures at start of treatment, and hence at end of treatment they remained more severe than the LO group. This has clinical implications, suggesting that more treatment sessions may be required for individuals who report an EO of symptoms.