This article reviewed the child and adult medical literature on the prevalence of comorbid anxiety disorders in patients with asthma. Theoretical ideas regarding the relatively high comorbidity rates are presented along with a model describing putative interactions between anxiety disorders and asthma. A search of the literature from the last 2 decades using MEDLINE by pairing the word, "asthma," with the following words: "anxiety," "depression," "panic," and "psychological disorders." We located additional research by screening the bibliographies of articles retrieved in the MEDLINE search. Both adult and child/adolescent populations with asthma appear to have a high prevalence of anxiety disorders. In child/adolescent populations with asthma, up to one third may meet criteria for comorbid anxiety disorders. In adult populations with asthma, the estimated rate of panic disorder ranges from 6.5% to 24%. However, most studies are limited by small samples, nonrepresentative populations, self-reported asthma status, and lack of controlling for important potential confounders such as smoking and asthma medications. There are also limited data on the impact of anxiety comorbidity in patients with asthma on symptom burden, self-care regimens (such as monitoring peak expiratory flow, taking medication, and quitting smoking), functional status, and medical costs. There appears to be a high comorbidity of anxiety disorders in patients with asthma. The prevalence and longitudinal impact of anxiety comorbidity needs to be examined in a large population-based sample of children, adolescents, and adults with asthma. If a high prevalence of comorbid anxiety disorder is documented and if this comorbidity adversely affects the self-efficacy and self-care, symptom burden, and functioning in persons with asthma, then it will be important to develop treatment trials.