Forty-nine obsessive-compulsive ritualisers completed a double-blind controlled study of clomipramine and exposure therapy. More severely ill patients allowed higher doses of medication to be prescribed and had higher plasma levels of both clomipramine and desmethylclomipramine. Exposure instructions had a strong effect, whereas the clomipramine effect was small and short-lived. Plasma levels of desmethylclomipramine but not of clomipramine correlated with outcome at weeks 8 and 17. There was no evidence of a therapeutic window for either clomipramine or its metabolite. Patients' physical complaints before treatment correlated positively with depression and anxiety, especially sexual difficulties. Dry mouth, as a side effect, was most evidently related to clomipramine and its usefulness in monitoring drug compliance for patients on clomipramine is reaffirmed.