An association between mental disorders, especially affective and anxiety disorders, and cancer has been reported in many studies. The present study investigated current (4-weeks-, 12-months-, and lifetime-prevalence rates of comorbid mental disorders in cancer patients. Through a cross-sectional design, 517 patients (75% female patients) from two acute inpatient care clinics, two rehabilitation clinics and nine specialised practices for oncology were examined with standardised scales for psychological burden and quality of life. Somatic parameters were assessed through standardised medical records. In the second-stage-examination, a sample of 200 patients was interviewed with standardised clinical interview (CIDI) in order to obtain DSM-IV diagnoses of mental disorders. Differences in the type of mental disorders were examined for gender, treatment setting, severity of cancer and physical impairment. Prevalence rates of mental disorders were 23.5% for the 4-weeks, 40% for the 12-months, and 56.5% for the lifetime periods. The current and 12-months rates of affective and anxiety disorders were approximately 25-33% higher than prevalence rates found in recent epidemiological studies of the general population. These higher rates were, however, mainly due to the preponderance of female patients with a higher risk for mental disorders compared with males. The most prevalent current disorders were affective (9.5%), and anxiety disorders (13%). Female gender was associated with an approximately 2-fold risk of mental disorders during the patient's lifespan. Current diagnosis of affective disorders in women was highly related to the cancer. Physical impairment was also associated with the frequency of current psychiatric disorders, especially affective and anxiety disorders. The frequency of mental disorders in cancer patients does not differ from results of recent international epidemiological studies of the normal population. The slightly higher rates of anxiety disorders are mainly due to phobias (simple, social and agoraphobia) without urgent need for treatment. A relatively large portion of patients, however, fulfil the criteria of minor depressive disorder which deserves clinical attention.