BACKGROUND Ulcerative colitis with diverticulosis (UCD), segmental colitis associated with diverticulosis (SCAD) and acute uncomplicated diverticulitis (AUD) may affect the same colonic regions, but the real incidence of these entities in clinical practice is unknown. OBJECTIVE To assess the incidence and the endoscopic findings of UCD, SCAD and AUD. METHODS From January 2004 to June 2009, 8525 consecutive colonoscopies were performed. Diagnosis of the diseases was based on specific endoscopic and histological (UCD and SCAD), and on endoscopic and radiological (AUD) patterns. RESULTS Ulcerative colitis with diverticulosis was diagnosed in 25 patients (0.3%), SCAD was diagnosed in 129 patients (2%) and AUD was diagnosed in 130 patients (2%). In UCD, the inflammation in colonic area harbouring diverticula always affects the overall colonic mucosa in all cases, involving also diverticular orifices. The endoscopic characteristic of SCAD is that inflammation is mainly detected within the inter-diverticular mucosa without involvement of the diverticular orifices. In AUD, the inflammation affects primarily diverticular orifice and peri-diverticular mucosa. CONCLUSIONS In clinical practice, the incidence of mucosal inflammation in the presence of colonic diverticular disease is low and endoscopy is the mainstay of differential diagnosis.