PURPOSE: The aim of this prospective study was to evaluate the effects of radiotherapy (RT) on psychiatric disorder in patients undergoing RT for head and neck cancer. MATERIALS AND METHODS: The prospective study included 51 patients with head and neck cancer. The diagnosis of psychiatric disorder was made by Diagnostic and Statistical Manual of Mental Disorders, 4th edition criteria. Severity of psychopathology was assessed by the Hamilton Depression Rating Scale (HAM-D). RESULTS: Although HAM-D score increased after RT (from 4 [0-26] to 7 [0-24]), this increase was not significant (P = 0.108). Fourteen (27.5%) of 51 patients had a psychiatric disorder before RT treatment; adjustment disorder in 6, depression in 4, sleep disorder in 3, anxiety disorder in 1 patient. On the other hand, 16 (31.4%) of 51 patients had a psychiatric disorder after RT treatment; adjustment disorder in 6, depression in 5, sleep disorder in 4, anxiety disorder in 1 patient. There was no significant difference between two periods in terms of the prevalence of psychiatric disorder (P = 0.721). CONCLUSIONS: The presence of psychiatric disorder was high in patients with head and neck cancer even before RT. Similarly, its high rate continued after RT. However, there is no significant effect of RT on development of psychiatric disorder.