OBJECTIVE To determine, using magnetic resonance imaging (MRI), the incidence of retropubic haematoma and any associated clinically significant effects after a xenograft (porcine dermis) sling (XS) or the tension-free vaginal tape (TVT) procedure. PATIENTS AND METHODS Between October 2003 and March 2004, 24 consecutive patients presenting with stress urinary incontinence (SUI) were enrolled in this prospective study; 12 each underwent an XS or TVT procedure. A vaginal balloon pack was used for only 3 h after XS and not after TVT. All patients had pelvic MRI 6-8 h after surgery. The primary outcome measure was the incidence and distribution of retropubic haematoma after each sling technique. Secondary outcome measures included the interval to the first three spontaneous voids, the bladder emptying efficiency of the first three voids, a visual analogue scale pain score at 24 h after surgery, and the short-term (6-month) cure rate for SUI. RESULTS Overall, six (25%) patients (four XS and two TVT) developed a retropubic haematoma. Most commonly, they spread along the right paravesico-urethral space between the right half of the levator ani and the bladder neck. Patients with large haematomas took significantly longer to void (median 14.5 vs 6.0 h, P = 0.048). There was no difference in pain score in patients with or with no haematoma. None of the patients had clinically detectable haematomas in the suprapubic wound. All six patients with haematomas were cured or improved at the 6-month follow-up. CONCLUSIONS MRI is a useful noninvasive method for detecting retropubic haematomas soon after surgery. There was a surprisingly high incidence of retropubic haematomas, especially after the XS procedure. Retropubic haematomas may influence postoperative voiding efficiency.