Background: The aim of this study was to measure the incidence of anxiety-depression comorbidity among a sample of prostate cancer (PCa) patients and to investigate the total score, factor score and scale item differences across subgroups defined according to the presence of anxiety and/or depression. Methods: 491 PCa patients who had received their initial diagnosis between 1 and 113 months previously completed a survey of background variables, as well as the Zung Self-Rating Anxiety and Depression Scales (SAS, SDS). Results: Incidence of anxiety-depression comorbidity was nearly 16%, and higher than that previously reported in other samples. Although the directional differences of total anxiety and depression scores were as expected, analysis of underlying factor structures suggested significant overlap between anxiety and depression symptomatologies, and this was supported by analysis of scale item differences across disorder subgroups. Conclusions: Comorbidity of anxiety and depression may be higher during treatment procedures and following treatment than has been previously suggested from data collected at the commencement of treatment for PCa. Diagnosis of anxiety or depression may benefit from consideration of symptoms that are more traditionally associated with the alternative disorder, thus potentially modifying treatment and support plans for PCa patients with these psychosocial disorders.