Several prostate cancer knowledge questionnaires exist but none have demonstrated both reliability and validity when used by men with the disease. This study aims to develop a reliable and valid knowledge questionnaire for men with prostate cancer. After developing a 40-item Prostate Cancer Knowledge Questionnaire (PCKQ-40) in phase I, it was piloted in phase II with 391 medical students. This resulted in the reduction of the tool by removing items of poor discriminatory value. A balance between true and false items and content domains was also maintained in the reduced scale. The PCKQ-12 had a moderate internal consistency. Phase III of the study assessed the reliability and construct validity of the tool by measuring the prostate cancer knowledge levels of men with prostate cancer (n=28), men with other cancers (n=10) and men without cancer (n=84). Men with prostate cancer achieved significantly higher PCKQ-12 scores compared with men without cancer, supporting the construct validity of the tool. The tool's reliability was also confirmed with a moderate internal consistency. This study provides some evidence for the reliability and validity of the PCKQ-12 and supports its use with men with prostate cancer in further research.