Objective: To study the preventive effect of a milk drink fermented with multistrain probiotics on antibiotic associated diarrhoea (AAD). Design: Double-blind placebo controlled study. Setting: University Hospital of North Norway. Subjects and methods: Of 853 patients treated with antibiotics, 87 met the inclusion criteria, and were randomized to ingestion of a fermented milk drink containing LGG, La-5 and Bb-12 (n ¼ 46) or placebo with heat-killed bacteria (n ¼ 41), during a period of 14 days. A diary was recorded, and stool samples were collected for microbiological analyses. Results: Sixty-three patients completed the study according to the protocol; two patients (5.9%) in the treatment group and eight (27.6%) in the placebo group developed AAD (P ¼ 0.035). The relative risk of developing AAD was 0.21 (95% confidence interval: 0.05-0.93) when given probiotic milk drink. Conclusion: A fermented multistrain probiotic milk drink may prevent four of five cases of AAD in adult hospitalized patients. Sponsorship: TINE BA, Oslo, Norway.