This study provides reliability, factorial validity, discriminant validity and criterion validity data of the Beck Anxiety Inventory (BAI) in a Spanish sample of 307 adult outpatients with psychological disorders. Factor analyses suggested that the BAI taps a general anxiety dimension composed of two related factors (somatic and affective-cognitive symptoms), but these factors hardly explained any additional variance beyond that accounted for by the full-scale score. Internal consistency estimate for the BAI was high (= 0.90). The BAI was correlated 0.63 with the Beck Depression Inventory-II, but a factor analysis of their items revealed two factors, suggesting that this correlation may be better accounted for by the relationship between anxiety and depression rather than by problems of discriminant validity. In regard to criterion validity, patients with anxiety disorders had higher scores on the BAI than the rest of the participating groups of patients, with the only exception of patients diagnosed with a depressive disorder. In sum, the Spanish version of the BAI is a reliable and valid instrument for detecting and quantifying anxious symptoms in patients with psychological disorders.