The present study examined whether trait anxiety as indexed by the Spielberger State-Trait Anxiety Inventory (STAI) can be employed as a predictor of behaviour therapy outcome. Thirty-six female spider phobic patients completed the STAI trait anxiety scale, and then received one 2.5 hours session of exposure in vivo treatment. Therapy outcome was evaluated by means of standardized self-report measures and a behavioural approach test. Results showed that STAI trait anxiety substantially contributed to therapy success on all outcome measures: the higher the trait anxiety scores, the less patients profited from behaviour therapy.