We investigated genetic factors in the determination of anxiety disorders in a study of 32 monozygotic (MZ) and 53 dizygotic (DZ) adult same-sexed twins. The frequency of anxiety disorders was twice as high in MZ as in DZ co-twins of the total proband group, alike in the MZ and DZ co-twins of the generalized anxiety disorder proband group, and three times as high in MZ as in DZ co-twins of the other proband groups. Anxiety disorders with panic attacks were more than five times as frequent in MZ as in DZ co-twins in a combined group of probands with panic disorders and agoraphobia with panic attacks. For generalized anxiety disorder, genetic factors were not apparent, while genetic factors were not apparent, while genetic factors appear to influence the development of the other anxiety disorders, especially panic disorder and agoraphobia with panic attacks.