BACKGROUND: Constipation is one of the most frequent disorders in Gastroenterology. Radiologic and physiologic studies are essential to the diagnosis and treatment of constipated patients. OBJECTIVE: We aimed to assess the usefulness of manometry and defecography in constipated patients with suspect of dyssynergic defecation. METHODS: Dyschesic patients referred for anorectal testing, all patients underwent anorectal manometry and defecography. RESULTS: Fifty eight patients consecutive patients referred from 1998 to 2003. According to manometric findings patients were classified in patients without dyssynergia (group I, n = 16) and patients with dyssynergia (group II, n = 42). Group I patients had intussusception (75%), abnormal rectal emptying (56%), rectocele > 2.5 cm (56%), perineal descending (50%) and inadequate opening of the anal canal (38%). Group II patients had inadequate opening of the anal canal (69%), rectocele < 2.5 cm (55%), perineal descending (17%), intussusception (45%) and abnormal rectal emptying (55%). Group I had a higher frequency of perineal descending (p = 0.01) and intussusception (p = 0.044). Group II had a higher frequency of inadequate opening of the anal canal (p = 0.03), and not for inadequate opening of the anorectal angle. CONCLUSIONS: Defecography showed disorders in group I that may contribute to symptomatology, and in group II the inadequate opening of the anal canal suggests the participation of different parts of the external anal sphincter other than puborectalis in the genesis of dyssynergia.