An acute-phase response (APR), manifested as an increase of acute-phase proteins has been shown in major depression. Panic disorder (PD) may share some aetiopathogenic mechanisms with depression, but APR has not been studied in this disorder. Forty-one panic patients in the first stages of their illness were compared with 32 healthy subjects of comparable sex, age, and body mass index. Clinical diagnosis was established with the Mini International Neuropsychiatric Interview, and severity with the Panic Disorder Severity Scale and the CGI scale. Laboratory determinations included four acute phase proteins (APPs) [albumin, gammaglobulins, fibrinogen, C-reactive-protein (CRP)] and basal cortisol level. Patients were studied after 8-wk follow-up taking selective serotonin reuptake inhibitors (SSRIs) to assess the evolution of the APPs. Gammaglobulin levels were lower, and both cortisol and CRP levels were higher in PD patients than in controls. APP did not differ between patients with or without agoraphobia. At follow-up, patients who responded to SSRIs presented a decrease in albumin levels, and a trend towards a decrease in cortisol and CRP compared with levels at intake. The conclusions of this study are that there is an APR in patients suffering from PD, and this APR tends to diminish after a successful treatment with SSRIs.