The spastic pelvic floor syndrome, caused by a paradox contraction of the sphincteric apparatus at defaecation instead of relaxing, leads to constipation with difficult evacuation. Forty patients (15 males and 25 females, average age 49 years, age range 15-78) affected by serious chronic idiopathic constipation, underwent, at our Institute, from June 1989 to September 1990, the following instrumental examinations: anal manometry; electromyography of the pelvic floor; proctogram; intestinal transit time; anorectal endoscopy; in addition, in 6 cases at risk for colorectal cancer, left colonoscopy. Fifteen patients showed dyskinetic functioning of the voluntary sphincteric apparatus. The following diagnostic methods proved to be of fundamental importance: proctogram, which revealed failure to open of the anorectal angle at defaecation (mean values: at rest 88.93 degrees +/- 6.62; at defaecation 88.93 degrees +/- 9.44); electromyography of the pelvic floor, which showed the anomalous contraction of the external anal sphincter. These patients were treated by means of an air inflated endoampullary balloon to evoke the sensation of a stool and its subsequent expulsion. The correct evacuating function was resumed definitely in 9 patients (60%); for the remaining 6 patients, regular sessions of re-education are still necessary. The spastic pelvic floor syndrome is a major cause of constipation and requires an accurate diagnostic method of investigating the correct functioning of the recto-pelvic region by means of the above-mentioned methods.(ABSTRACT TRUNCATED AT 250 WORDS)