Diarrhea, urgency, and fecal incontinence are common complaints in systemic mastocytosis and in patients with increased gastrointestinal mucosal mast cells. We performed anorectal manometry on six patients with clinical symptoms of mastocytosis and histologic evidence of increased mast cells and compared the results to anorectal manometry of six age- and sex-matched controls, with no bowel symptoms. Standard techniques with balloon volumes were used to measure maximal basal pressure, maximal squeeze pressure, smallest volume sensed, degree of relaxation of the internal sphincter, and the volume causing: (1) a strong urge to defecate and (2) pain. Patients with mastocytosis, compared with controls, had smaller balloon volumes induce rectal urgency (97 vs 164 ml) and pain (117 vs 278 ml). A trend was present for lower maximal basal pressure in mastocytosis, but was not statistically significant. Sensitivity to balloon inflation suggests decreased rectal compliance or overreactive rectal contractility. These findings provide an explanation for the anorectal symptoms in patients with increased mast cells.