An association between mental disorders, especially affective and anxiety disorders, and cancer has been reported in many clinical studies with inpatients. However, no data exist about the prevalence of mental disorders in patients undergoing acute care or rehabilitation treatment for cancer in Germany. The present study investigated 4-week, 6-month and lifetime prevalence rates of comorbid mental disorders in cancer. 256 patients from 2 rehabilitation clinics and 2 acute care hospitals were examined with standardized screening scales for psychological burden (GHQ-12, HADS) and quality of life (SF-36). Somatic parameters and interventions were assessed through standardized medical records from the attending oncologists. In the second-stage examination a subsample of 120 patients were interviewed with standardized clinical interview (CIDI) in order to obtain DSM-IV diagnoses of mental disorders. 44% (acute care) and 49% (rehabilitation) of the patients have high GHQ scores (cut-off > 4). Furthermore, more than 20% of the cancer patients have elevated scores on the HADS subscales depression and anxiety (cut-off > or = 11). Prevalence rates of mental disorders in the rehabilitation sample are 34% for the 4-week (vs. 24% in the acute care sample), 45% for the 6-month period (vs. 38%), and 79% for lifetime (vs. 49% in the acute care sample). The most frequent current disorders are affective (13% rehabilitation vs. 11% acute care) and anxiety disorders (17% in both samples). The rates of affective and anxiety disorders are much higher than the frequency of these disorders in recent epidemiological studies of the normal population in Germany. The high psychosocial burden expressed by the patients and the frequency of depressive and anxiety disorders emphasize the importance of (1) effective diagnostic strategies to recognize mental disorders, and (2) specialized psychosocial services in oncology, which provide psychological support and effective interventions for cancer patients in acute care and rehabilitation.