Voiding difficulty is a common complaint during a few days after implantation of the penile prosthesis. We measured Maximum Flow Rate(MFR) before and 7 days after implantation of the penile prosthesis in 78 patients. The average age of the patients was 43.3 years with a range of 21 to 70 years. The type of the implanted prosthesis was malleable prosthesis in 38 patients(AMS 600), inflatable prosthesis in 20(AMS 700CX) and Hydroflex prosthesis in 20. We investigated postoperative changes of MFR in accordance with age, type of the implanted prostheses and underlying disease of the existing voiding difficulty. Among 26 cases who had existing voiding problems, 14 were urethral stricture, 4 benign prostatic hyperplasia and 8 neuogenic bladder. There was significant difference between preoperative and postoperative MFR in the group of malleable penile prosthesis(p<0.05), but it was minimal decrease less than 5ml/sec. In conclusion, implantation of malleable penile prostheses could have at least minimally bad influence on the urinary flow rate.