In this study, the influencing factors of different durations of in-patient therapy were examined. We investigated 1173 patients with anxiety disorders who were treated in the psychosomatic clinic of Bad Pyrmont with behavioural therapy. Patients with anxiety disorders have an average treatment duration of 52.4 days with a minimum of less than one week and a maximum of more than five months. The duration of treatment depends on the one hand on sociodemographic variables such as age, marital status, partnership, residential status, and professional activity, but not on gender or education. The duration of treatment also depends on factors of severity of the disorder, such as the number of in-patient or psychotherapeutic treatments, duration of disorder and disability, and the number of psychiatric diagnoses, but not on the number of somatic diagnoses. There is a positive correlation between duration of treatment and effect of therapy as well as recovery of fitness for work.