AIM: Small intestinal bacterial overgrowth (SIBO) may contribute to the appearance of several gastrointestinal nonspecific symptoms. Acute diverticulitis is affected by some similar symptoms and bacterial colonic overgrowth. We assessed the prevalence of SIBO in acute uncomplicated diverticulitis and evaluated its influence on the clinical course of the disease. METHODS: We studied 90 consecutive patients (39 males, 51 females, mean age 67.2 years, range 32-91 years). Sixty-one patients (67.78%) and 29 patients (32.22%) were affected by constipation-or diarrhea-prevalent diverticulitis respectively. All subjects were investigated by lactulose H_2-breath test at the entry and at the end of treatment. We also studied a control group of 20 healthy subjects (13 males, 7 females, mean age 53 years, range 22-71 years). RESULTS: Oro-cecal transit time (OCTT) was delayed in 67/90 patients (74.44%) (range 115-210 min, mean 120 min). Fifty-three of ninety patients (58.88%) showed SIBO, while OCTT was normal in 23/90 patients (25,56%). In the control group, the mean OCTT was 88.2 min (range 75-135 min). The difference between diverticulitic patients and healthy subjects was statistically significant (P＜0.01). OCTT was longer in constipation-prevalent disease than in diarrheaprevalent disease [180.7 min (range 150-210 min) vs 121 min (range 75-180 min) (P＜0.001)], but no difference in bacterial overgrowth was found between the two forms of diverticulitis.After treatment with rifaximin plus mesalazine for 10d, followed by mesalazine alone for 8 wk, 70 patients (81.49%) were completely asymptomatic, while 16 patients (18.60%) showed only slight symptoms. Two patients (2.22%) had recurrence of diverticulitis, and two other patients (2.22%) were withdrawn from the study due to side-effects. Seventy-nine of eighty-six patients (91.86%) showed normal OCTT (range 75-105 min, mean 83 min), while OCTT was longer, but it was shorter in the remaining seven (8.14%) patients (range 105-115 min, mean of 110 min). SIBO was eradicated in all patients, while it persisted in one patient with recurrence of diverticulitis. CONCLUSION: SIBO affects most of the patients with acute diverticulitis. SIBO may worsen the symptoms of patients and prolong the clinical course of the disease, as confirmed in the case of persistence of SIBO and diverticulitis recurrence. In this case, we can hypothesize that bacteria from small bowel may re-colonize in the colon and provoke recurrence of symptoms. AIM: Small intestinal bacterial overgrowth (SIBO) may contribute to the appearance of several gastrointestinal nonspecific symptoms. Acute diverticulitis is affected by some similar symptoms and bacterial colonic overgrowth. We assessed the prevalence of SIBO in acute uncomplicated diverticulitis and as its influence on the clinical Sixty-one patients (67.78%) and 29 patients (32.22%) were affected by constipation -or diarrhea-prevalent diverticulitis respectively. All subjects were investigated by the lactulose H 2-breath test at the entry and at the end of treatment. We also studied a control group of 20 healthy subjects (13 males, 7 females, mean age 53 years, range 22-71 years). RESULTS: Oro-cecal transit time (OCTT) was delayed in 67/90 patients (74.44%) (range 115-210 min, mean 120 min). Fifty-three of ninety patients showed SIBO, while OCT In the control group, the mean OCTT was 88.2 min (range 75-135 min). The difference between diverticulitic patients and healthy subjects was statistically significant (P <0.01). T was normal in 23/90 patients (25,56% . OCTT was longer in constipation-prevalent disease than in diarrheaprevalent disease [180.7 min (range 150-210 min) vs 121 min (range 75-180 min) (P <0.001)], but no difference in bacterial overgrowth was found between the Two forms of diverticulitis. After treatment with rifaximin plus mesalazine for 10d, followed by mesalazine alone for 8 weeks, 70 patients (81.49%) were completely asymptomatic, while 16 patients (18.60%) showed only slight symptoms. Two patients (2.22%) Nine recurs of diverticulitis, and two other patients (2.22%) were withdrawn from the study due to side-effects. Seventy-nine of eighty-six patients (91.86%) showed normal OCTT (range 75-105 min, mean 83 min) , while OCTT was longer, but it was shorter in the remaining seven (8.14%) patients (range 105- 115min, mean of 110 min). SIBO was eradicated in all patients, while it persisted in one patient with recurrence of diverticulitis. CONCLUSION: SIBO affects the of most patients with acute diverticulitis. SIBO may worsen the symptoms of patients and prolong the clinical course of the disease, as confirmed in the case of persistence of SIBO and diverticulitis recurrence.