BACKGROUND: Major depression is found in around ten percent of patients attending cancer clinics. One of the symptoms of major depression, defined in the Diagnostic and Statistical Manual of Mental Disorders (DSM), is "thoughts of death or suicide." To implement depression screening programs for patients with cancer, we need to know the prevalence not only of this broadly defined symptom but also of more specific suicidal thoughts, as only the latter clearly indicates the need for specialist psychiatric assessment and management of suicide risk. METHODS: We analyzed data from a routine 2-stage depression screening program that operated in 3 UK cancer centers, linked with demographic and clinical data obtained from a national cancer registry. We included data on 2217 patients with common cancers and comorbid, interview-diagnosed, major depression. We determined the percentage of patients with (a) the DSM-IV symptom "thoughts of death or suicide" and (b) suicidal thoughts, defined as an affirmative response to the question "have you had thoughts of ending your life?" We investigated the associations of patients' demographic and clinical characteristics with each of these using logistic regression models. RESULTS: We found that 641 of 2217 (29%) of patients had the DSM symptom "thoughts of death or suicide" and 207 of 2217 (9%) had suicidal thoughts. Of the demographic and clinical characteristics that we studied, none had statistically significant associations with having the DSM symptom. Only younger age and primary cancer were associated with having suicidal thoughts. CONCLUSIONS: We found that almost one-third of patients with cancer and comorbid major depression have the DSM symptom "thoughts of death or suicide." However, only a third of the patients with this symptom report suicidal thoughts. These findings suggest that around one in ten patients found by a screening program to have major depression will have suicidal thoughts requiring a psychiatric assessment. The staffing of depression screening programs should be designed with these data in mind.