Background: To study the clinical relevance of type of comorbidity and number of comorbid disorders in anxiety disorders. Four groups were compared according to sociodemographic-, vulnerability- and clinical factors: single anxiety disorder, anxiety-anxiety comorbidity, anxiety-depressive comorbidity and "double" comorbidity (i.e. anxiety and depressive comorbidity). Methods: Data were obtained from the Netherlands Study of Anxiety and Depression (NESDA). A sample of 1004 participants with a current anxiety disorder was evaluated. Results: As compared with single anxiety, anxiety-anxiety comorbidity was associated with higher severity, greater chronicity and more treatment. Anxiety-anxiety comorbidity was associated with an earlier age of onset and a more chronic course compared with anxiety-depressive comorbidity, while anxiety-depressive comorbidity was associated with more severe symptoms and more impaired functioning than anxiety-anxiety comorbidity. "Double" comorbidity was associated with higher severity, greater chronicity, more treatment and increased disability. Sociodemographic and vulnerability factors were comparable among the four groups. Limitations A prospective design would be more appropriate to study the outcome. In this study no distinction was made between whether depression or anxiety disorder preceded the current anxiety disorder. Conclusions: It is clinical relevant to diagnose and treat comorbidity among anxiety disorders as it is associated with higher severity and more chronicity. Whereas anxiety-anxiety comorbidity has an earlier age of onset and a more chronic course, anxiety-depressive comorbidity leads to more treatment and impaired functioning. "Double" comorbidity leads to even more severity, chronicity and impairment functioning compared with both anxiety-anxiety and anxiety-depressive comorbidity. © 2011 Elsevier B.V. All rights reserved.