Introduction and hypothesis The objective of our study was to estimate the incidence and to identify the risk factors for reoperation of surgically treated pelvic organ prolapse (POP). Methods We conducted a nested case-control study among 1,811 women who underwent POP surgery from January 1988 to June 2007. Cases (n=102) were women who required reoperation for POP following the first intervention through December 2008. Controls (n=226) were women randomly selected from the same cohort who did not require reoperation. Results The incidence of POP reoperation was 5.1 per 1,000 women-years. The cumulative incidence was 5.6%. Risk factors included preoperative prolapse in more than two vaginal compartments (adjusted OR 5.2; 95% CI 2.8-9.7), history of surgery for POP and/or urinary incontinence (adjusted OR 3.2; 95% CI 1.5-7.1), and sexual activity (adjusted OR 2.0; 95% CI 1.0-3.7). Conclusions The risk of POP reoperation is relatively low and is associated with preexisting weakness of pelvic tissues.