We compared erect penile length after inflatable penile prosthesis (IPP) implantation with that induced by in-tracavernosal injection (ICI) before surgery. A total of 11 patients with full erections induced by ICI of 0.25 mL of Trimix (papaverine, phentolamine and prostaglandin E1) at the time of penile color duplex Doppler ultrasonography were enrolled in this study. The patients subsequently underwent IPP implantation as the ultimate treatment for their erectile dysfunction (ED). Erect penile length consequent to IPP implantation was measured at 6 weeks, 6 months and at 1 year after surgery. The Sexual Health Inventory for Men (SHIM) was administered before, and at 6 months and 1 year after IPP implantation. Erect penile length (mean +/- s.e.) as induced by ICI was 13.2 +/- 0.4 cm, whereas the lengths attained with IPP were 12.4 +/- 0.3, 12.5 +/- 0.3 and 12.5 +/- 0.4 cm at the sixth week, sixth month and 1-year follow-ups, respectively. There were 0.83 +/- 0.25, 0.75 +/- 0.20 and 0.74 +/- 0.15 cm decreases in erect penile length at 6 weeks, 6 months and 1 year, respectively, after IPP implantation when compared with that after ICI (P < 0.05). The SHIM scores for patients reporting shorter penises were the same as those for patients without complaints at the 6-month and 1-year follow-ups (P > 0.05). To our knowledge, this is the first study to objectively show a significant decrease in erect penile length after IPP implantation when compared with that after ICI. However, this decrease did not affect the effectiveness of IPP in treating ED.