Purpose: This study was done to explore the relationship of social support and meaning of life to suicidal thoughts among patients with cancer. Methods: Data were collected by questionnaires from 138 patients who had been in cancer treatment at medical clinics and 8 patients who were members of an internet cancer association. The data were collected between August and November 2009 and analyzed using SPSS 12.0. Results: Of the participants, 47.3% reported having had suicidal thoughts and 16.4% had attempted suicide since the diagnosis of cancer. The study participants received most support from family members, but 73.3% reported experiencing an existential vacuum. The suicide attempt group had significantly higher scores according to gender, age, level of education, diagnosis, treatment modality, level of activity, caregiver and social support compare to the suicide thought group. Suicidal thoughts were negatively related to social support and meaning of life was positively associated with social support. Support from family and friends and diagnosis explained 50.0% of variance for suicidal thoughts with 36.0% of variance being explained by family support. Conclusion: Nurses should be able to identify risk factors for suicide in cancer patients. Prevention and intervention efforts need to be directed toward improving social support, family support in particular, and assisting patients finding meaning in life after a diagnosis of cancer.