A placebo-controlled double-blind study was conducted on the efficacy of Lactobacillus GG in preventing travellers' diarrhoea. Altogether 820 persons travelling on holiday to southern Turkey to two destinations were randomized into two groups receiving either Lactobacillus GG or placebo in identical sachets. On the return flight each participant completed a questionnaire indicating the incidence of diarrhoea and related symptoms during the trip. Of the original group 756 (92%) subjects completed the study acceptably. The overall incidence of diarrhoea was 43.8% (331 cases). The total incidence of diarrhoea in the placebo group was 46.5% and in the Lactobacillus GG 41.0% indicating an overall protection of 11.8%. Protection rates varied between two different destinations with the maximum protection rate reported as 39.5%. Among older age groups there was significantly less diarrhoea when compared to younger travellers. Lactobacillus GG appeared to be effective in reducing the occurrence of travellers' diarrhoea in one of the two destinations with no side effects.