The present invention relates to a method of diagnosing, or predicting risk of, prostate disease in a subject. More particularly, the invention relates to a method of diagnosing the presence of, or the predisposition to develop, prostate disease in a subject, the method comprising analysing a test sample from the subject for the presence of P. acnes infection of the prostate gland. The present invention further relates to reagents for use in this method and to methods of prevention or treatment of prostate disease.