The objective of this study was to ascertain if incomplete correction leaving a residual uterine septum of < or = 1 cm affects fertility outcome. Reproductive outcome in 17 women with a residual septum of between 0.5 cm and 1 cm after hysteroscopic metroplasty was compared to that in 51 women with no residual septum or one of < 0.5 cm. Septal lysis was performed with microscissors or resectoscope. One month after operative hysteroscopy, abdominal ultrasonography was performed on all the women and those with a residual septum of > 1 cm then underwent a second operative hysteroscopy to complete the lysis. The cumulative pregnancy and birth rates were calculated and the curves compared using the log-rank test. The cumulative 18 month probability of becoming pregnant was 44.5% in the patients with residual septum and 52.7% in those with no residual septum (not significantly different), and the cumulative 18 month probability of giving birth to a child was 27.5 and 36% respectively (also not significant). The presence of a residual uterine septum of between 0.5 and 1 cm as shown by ultrasonography appears not to worsen the reproductive prognosis compared with that in women in whom the septum has been completely or almost completely corrected.