The present study examined whether trait anxiety as indexed by the SpielbergerState-Trait Anxiety Inventory (STAI) can be employed as a predictor ofbehaviour therapy outcome. Thirty-six female spider phobic patients completedthe STAI trait anxiety scale, and then received one 2.5 hours session ofexposure in vivo treatment. Therapy outcome was evaluated by means ofstandardized self-report measures and a behavioural approach test. Resultsshowed that STAI trait anxiety substantially contributed to therapy success onall outcome measures: the higher the trait anxiety scores, the less patientsprofited from behaviour therapy.