The longitudinal association of several syndromal diagnoses is very frequently a direct consequence of modern descriptive diagnosis. Comorbidity in this sense is clinically relevant. Comorbid cases are more severe, are more amenable to treatment and are more frequently suicidal. The level of association between psychiatric syndromes can lead to nosologic hypotheses that can be further examined by independent investigations, and especially by means of family studies. Generalized anxiety disorders are very closely associated with the affective disorders, particularly with depressions and suicide attempts, but also with hypomania. There is no close relationship with panic disorder. Social phobias are highly associated with agoraphobia, but also with simple phobia; also with panic, obsessive-compulsive syndromes and substance abuse. The prevalence of obsessive-compulsive syndromes depends to an exceptional degree on the definition. Syndromes below the diagnostic threshold of DSM-III are extremely frequent, and longitudinally a fluctuation about this threshold is apparent. OCS are especially found to be associated with social phobia and agoraphobia as well as with dysthymia and recurrent brief depression, but less with major depression.