Objectives: Depression is common in cancer patients. However, only limited evidence is available for Asian populations. The authors therefore examine the prevalence of depression in Thai patients with cancer. In addition, associated factors were determined. Methods: This cross-sectional study was conducted in cancer patients admitted to a university hospital during December 2006 - December 2007. The Patient Health Questionnaire (PHQ-9) was used to assess all cancer patients. Suicidal risk was assessed by using the Mini-International Neuropsychiatric Interview (MINI) in the module of suicidal risk assessment. Results: Of 108 cancer patients, 29.6 % were diagnosed with a depressive disorder (mild, 14.8 %; moderate, 5.6 %; severe, 9.3 %). However, only 25.0 % of these were recognized as being depressed by the primary physician. According to the MINI., 28.1 % of these depressed cancer patients had a moderate to severe level of suicidal risk. In addition, the findings suggest that increased risk of depression is significantly associated with increased pain score, lower number of cancer treatments (< 2 methods), increased educational duration (>13 years), increased age (> 50 years old) and being female. Conclusions: The prevalence of depression is high in Thai cancer patients. However, depressive disorder in those patients is frequently undiagnosed. It is associated with several factors including pain, a number of cancer treatments, education duration, age and sex. To improve quality of life, increase compliance with treatments and prevent of suicide, screening for depressive disorders in this patient group is strongly recommended.