BACKGROUND: Although interest in sphincter-sparing treatments for anal fistulas is increasing, few large prospective studies of these approaches have been conducted. OBJECTIVE: The study assessed outcomes after implantation of a synthetic bioabsorbable anal fistula plug. DESIGN: A prospective, multicenter investigation was performed. SETTING: The study was conducted at 11 colon and rectal centers. PATIENTS: Ninety-three patients (71 men; mean age, 47 years) with complex cryptoglandular transsphincteric anal fistulas were enrolled. Exclusion criteria included Crohn's disease, an active infection, a multitract fistula, and an immunocompromised status. INTERVENTION: Draining setons were used at the surgeon's discretion. Patients had follow-up evaluations at 1, 3, 6, and 12 months postoperatively. MAIN OUTCOME MEASURES: The primary end point was healing of the fistula, defined as drainage cessation plus closure of the external opening, at 6 and 12 months. Secondary end points were fecal continence, duration of drainage from the fistula, pain, and adverse events during follow-up. RESULTS: Thirteen patients were lost to follow-up and 21 were withdrawn, primarily to undergo an alternative treatment. The fistula healing rates at 6 and 12 months were 41% (95% CI, 30%-52%; total n = 74) and 49% (95% CI, 38%-61%; total n = 73). Half the patients in whom a previous treatment failed had healing. By 6 months, the mean Wexner score had improved significantly (p = 0.0003). By 12 months, 93% of patients had no or minimal pain. Adverse events included 11 infections/abscesses, 2 new fistulas, and 8 total and 5 partial plug extrusions. The fistula healed in 3 patients with a partial extrusion. LIMITATIONS: The study was nonrandomized and had relatively high rates of loss to follow-up. CONCLUSION: Implantation of a synthetic bioabsorbable fistula plug is a reasonably efficacious treatment for complex transsphincteric anal fistulas, especially given the simplicity and low morbidity of the procedure.