BACKGROUND Penile abscesses were traditionally regarded as an infectious disease; however, idiopathic cases in which prednisolone was effective have been reported. CASE REPORT A 64-year-old man was admitted to the hospital with symptoms of penile induration and dysuria. He was diagnosed with a penile abscess, which was punctured and then relapsed. An incision and drainage were performed on the abscess, and the pus and tissue samples were cultured and examined histologically. There was no evidence of malignancy or bacterial infection, and he was diagnosed with an idiopathic penile abscess. As pus continuously drained from the incision, prednisolone 40 mg was initiated, which resulted in a decreased amount of pus and eventual wound closure. Over 15 months, prednisolone was gradually tapered to 5 mg, and the abscess continued to decrease in size. CONCLUSIONS Idiopathic penile abscesses are rare but often lead to penectomy. Prednisolone is a new treatment method for such patients. This is the third case of an idiopathic penile abscess that was successfully treated with prednisolone. The causative agent of the idiopathic penile abscess was suggested to be pyoderma gangrenosum; however, this case did not exhibit the typical characteristics of pyoderma gangrenosum. Therefore, further investigation was needed. A differential diagnosis of an infectious abscess is required before initiating steroid treatment. Open drainage is useful, but the size of the incision should be minimized for the purpose of preserving penile function. The prednisolone dose should be started at 20 to 40 mg and reduced gradually to avoid relapse.