Objective: Depression among prostate cancer patients remains a major source of distress for them and their families, has been linked with suicide, and has been shown to contribute to poorer longterm treatment outcomes. Most psychological assessment strategies focus upon the presence of depressive symptomatology without identifying the specific causal antecedents that lead to depression among this patient group, although this underlies effective individually-oriented treatment planning and delivery. Method: 150 prostate cancer patients completed self reports on anxiety, depression and lifestyle changes that they had experienced as a result of receiving a diagnosis of, and treatment for their cancer. The principal instrument being investigated was a measure of 50 lifestyle changes that had been developed from previous interviews with prostate cancer patients. Results: Data indicated significant relationships between depression scores and the frequency of unpleasant lifestyle changes and the ratings that participants gave to these changes, supporting a model of depression as an adaptive withdrawal from noxious stimuli and environments. Psychometric data provided a method of deleting several items so that a clinical analysis and a research scale of 36 items was developed for use with prostate cancer patients. Conclusion: Implications for treatment are discussed in terms of accurate identification of antecedents that lead to a further understanding of depression as an 'adaptive' response of active withdrawal from noxious environments.