Depression is the most common comorbid disor¬der in OCD and the best predictor of low QoL in OCD according to most studies (see above). Between one third44,45 and two thirds of OCD patients46 are clinically depressed. Apter and colleagues47 found that almost 50% of adolescent OCD patients display scores of >30 on the Beck Depression Inventory, which equates to very severe depression. The pathogenetic relationship between depression and OCD is complex and not yet fully elucidated. For example, obsessions and depressive symptoms are highly correlated30 and OCD worries and depressive ruminations are often almost inseparable. While most depressive symptoms are secondary to OCD, the opposite, primary depression and later onset of OCD, have also been described. An older study45 estimated that 37% of their OCD sample showed secondary depression while 29% had OCD symp¬toms that followed a primary depression. Twenty-one percent did not suffer from depression and for 13% no decision was possible.