OBJECTIVE The aim of this study was to determine whether overactivity of the anal sphincter in patients with hemorhhoids is primary or secondary and thus assess indication of lateral internal sphincterotomy to surgical treatment of hemorrhoids. Tonic contraction of the sphincter muscle in patients with advanced stages of hemorrhoids is considered by many authors as a primary cause, and therefore, they complete hemorrhoid surgery with lateral internal sphincteroomy. If hypertension of anal sphincter is secondary during hemorrhoid disease, lateral internal sphincterotomy is not indicated. Although examinations made immediately after sphincterotomy proved no changes of anal continence, certain sequelae of lateral internal sphincterotomy cannot be excluded and may later negatively affect patient's anal continence. METHODS The prospective study comprised 385 patients treated in 2002-2006 by Hemoron or surgery according to Milligan-Morgan or Longo. Patients with history of another disease of the anal canal, radiotherapy of pelvis, Crohn's disease or ulcerous colitis were excluded. Manometry was performed before and after surgery at intervals of 1, 3, 6 and 12 months after operation using a perfusion flow method, six-channels catheter with radial arrangement of channel tips. RESULTS In all three groups (Hemoron, sec. Milligan-Morgan, sec. Longo), there were 60-65% of patients with third degree hemorrhoids. Normal resting anal pressure before surgery was recorded in only 25% of men and 30% of women. Patients with advanced hemorrhoid degrees were found to have significant hypertension of the anal sphincter. The most significantly improved state of sphincter overactivity was observed after surgery according to Longo and application of Hemoron. After surgery, according to Milligan-Morgan, recovery of anal sphincter tension was the longest; even 6 months after operation, a mean increased resting anal pressure persisted (91-110 mmHg) in 25% of men and 19% of women. After 12 months, recovery of anal tension occurred in this group also--mean increased anal pressure was recorded in only three patients (1.67%). CONCLUSIONS Overactivity of the anal sphincter in patients with hemorrhoids is secondary and according to our results. Hypertension of the sphincter muscle in patients with hemorrhoids is significantly increased in patients with advanced degrees of hemorrhoids. Therefore, it is not recommended to postpone surgery and indicate patients with advanced degrees of hemorrhoids to hemorrhoidectomy.