OBJECTIVE Perianal disease is frequent in patients with Crohn's disease, and many of these patients will eventually have abscess formation. In a prospective follow-up study, we evaluated factors influencing the occurrence and recurrence of perianal abscesses. METHODS Of 126 consecutive patients with perianal Crohn's disease seen regularly in an outpatient clinic, 61 (48.4 percent) had at least one perianal abscess (mean follow-up, 32 +/- 17 months). In all, 110 episodes of an abscess with 145 anatomically distinct abscesses were documented. RESULTS The occurrence of first abscesses was dependent on the type of anal fistula (ischiorectal, 73 percent; transsphincteric, 50 percent; superficial, 25 percent; P < 0.02). Surgical therapy consisted of seton drainage (34 percent), mushroom catheter drainage (49 percent), or incision and drainage (29 percent) and led to inactivation in all patients. Cumulative two-year recurrence rates after the first and second abscess were 54 and 62 percent, respectively. Abscess recurrence was less frequent in patients with a stoma (13 vs. 60 percent in patients without stoma after two years) and in patients with superficial anal fistulas (0 vs. 55 percent/56 percent in patients with transsphincteric/ischiorectal fistulas). Only two abscesses recurred within one year after removal of seton drainage, whereas 13 abscesses recurred with the seton still in place. Neither intestinal nor rectal activity of Crohn's disease significantly influenced the occurrence of an abscess. During the study period, only two patients developed partial stool incontinence. CONCLUSIONS Development of perianal abscesses in Crohn's disease depends on the fecal stream and the anatomic type of anal fistula. Seton and catheter drainage are safe and highly effective in treatment. Long-term use of setons to prevent recurrent abscesses is not supported by our data.