This article has reviewed clinical and demographic features of the primary anxiety disorders and other psychiatric and medical disorders that often are associated with anxiety symptoms, highlighting differential diagnosis. In summary, phobic disorders (exogenous anxiety) are characterized by anxiety reliably elicited by specific environmental stimuli; the stimuli involved determine which type of phobia is diagnosed. In contrast, panic attacks and generalized anxiety (endogenous anxiety) involve symptoms of anxiety not associated only with specific eliciting stimuli. Panic disorder is differentiated from generalized anxiety disorder by the presence of discrete attacks; both disorders usually have some level of persistent anxiety. Obsessive-compulsive disorder is characterized by recurrent unwanted but irresistible thoughts and the ritualized repetitive acts resulting from these obsessions, in the absence of preexisting psychosis or depression. Finally, posttraumatic stress disorder involves various anxiety (and other) symptoms as a direct result of an obvious stressor. Depressive symptoms are frequently associated with anxiety. It is sometimes impossible to determine which is the primary disorder. Overlap of syndromes probably also occurs with other primary psychiatric disorders, especially somatoform disorders, adjustment disorder with anxious mood, and several personality disorders. Finally, primary anxiety can be confused with several medical syndromes, especially when the medical disorder has not been recognized. Nevertheless, research with patients with pheochromocytoma suggests that medical causes of anxiety may be qualitatively different from primary anxiety disorders, especially the psychic anxiety component. Attention to the clinical and demographic features listed in Table 4, as well as the use of newly-developed structured diagnostic interviews should usually lead to a correct diagnosis, as illustrated by the following examples. The onset of a fear of public speaking in mid-adolescence suggests an uncomplicated social phobia, whereas the onset in the mid-twenties of several social and other situational anxieties in a person with a previous history of panic attacks would be strongly suggestive of the panic-agoraphobia syndrome. The new onset of generalized anxiety symptoms and depression in a 45-year-old patient who has had a previous significant depression would suggest that this person's anxiety is part of, and secondary to, the affective disorder and not a primary anxiety disorder.(ABSTRACT TRUNCATED AT 400 WORDS)