Diarrhoea is a severe side-effect of radiotherapy on the pelvic area. It is due to acute enteric damage. We aimed at determining the ability of a highly concentrated freeze-dried living bacteria compound (VSL/3) to reduce these side-effects in 190 patients receiving radio therapy on the pelvic area. A total of 95 patients received radiotherapy alone and 95 were also administered VSL/3 bags, at doses of one bag three times a day beginning on the first day of the radiotherapy treatment. The same diet was indicated for both groups. All patients were irradiated for 6 to 7 weeks, with Linac X-6 MV or 15 MV through a box multiportal technique with the lower limit of the fields below the obturator foramina, upper limit at L5-S1, lateral limit 1.5 cm beyond the innominate hip. The total radiated dose ranged from 60 to70 Gy for a daily dose of 180 cGy. Gastroenteric toxicity was rated in WHO degrees. Two patients receiving radiotherapy alone had to discontinue the treatment due to acute enteritis. Toxicity was found in 52 (50.6%) patients with radiotherapy alone vs 36 (30.5%) patients receiving VSL/3. None of them had to discontinue radiotherapy. Toxicity of degrees 3 or 4 was found in 28 patients receiving radiotherapy alone vs 7 with VSL/3. These preliminary data suggest the effectiveness of VSL/3 in preventing the occurrence of diarrhoea in patients submitted to radiotherapy with a direct and indirect improvement of their quality of life and a good tolerance.