Congenital megacaecum is a rare entity and difficult to diagnose. The pathogenesis of this malformation is not well known since there are very few cases reported in the literature. The purpose of this observation is to describe the functional signs that may suggest this rare diagnosis and the means to confirm it. We report the case of a 22-year-old young man, who complained of constipation associated with pelvic pain in the form of gravitational pull exacerbated by standing, sitting and going down the stairs. However, symptoms seemed to be relieved by supine positions. The radiological investigations concluded that the megacaecum dipped into the pelvis, but there was no evidence of mechanical or functional obstruction distally. The treatment consisted of a laparoscopic right colectomy. The postoperative course was uneventful. The megacaecum is rare and poorly understood. The abdominal pain is directly related to fecal stasis, which increases the pressure on cecal colonic segment and this in turn causes a pull on the mesentery. The entero-MRI is valuable in the diagnosis so as to eliminate other causes of chronic abdominal pain.