PURPOSE:Controversy exists as to whether fissure patients have elevated resting pressures when compared with control patients. The diameter of manometry catheters used in past studies varies widely (1.5‐25 mm) and may have contributed to differences observed in resting pressures. A prospective study was undertaken to determine the influence of manometry catheter diameter on maximum resting pressure in patients with idiopathic chronic anal fissures.METHODS:A total of 28 fissure patients and 28 control patients had manometry performed with both a 1.8‐mm and a 4.8‐mm (external diameter) water‐perfused catheter.RESULTS:Mean maximum resting pressure (RP) for fissure patients as measured with the 1.8‐mm catheter was 86 (range, 65‐115) mmHg and 83 (range, 47‐117) mmHg with the 4.8‐mm catheter(P=0.65). Mean maximum RP for control patients with the 1.8‐mm catheter was 70 (range, 30‐108) mmHg and 72 (range, 35‐109) mmHg with the 4.8‐mm catheter(P=0.07). When fissure and control patients were compared, a significantly higher mean RP was observed in the fissure group for both the 1.8‐mm catheter (86vs.70 mmHg, respectively;P=0.01) and the 4.8‐mm catheter (83vs.72 mmHg, respectively;P=0.03). There was no significant difference in length of the high‐pressure zone within each group or when the fissure group and controls were compared, regardless of catheter used. For both groups of patients, there was a significantly higher incidence of ultraslow waves (USWs) observed with the 4.8‐mm catheter when compared with the 1.8‐mm catheter. The USW frequency was not significantly different when fissure and control groups were compared with either catheter type.CONCLUSIONS:Catheter size did not influence measured maximum RP in fissure patients. The maximum RP was significantly greater for fissure patients overall when compared with the control group, regardless of catheter used. There was an increased frequency of USWs noted with the larger catheter size in all patients; however, these USWs only became apparent when catheter was left at each station until a true baseline RP was obtained.