To determine the effect of a human Lactobacillus strain GG (Gefilac™) on the clinical outcome and recovery from mucosal damage of acute diarrhoea (82 % rotavirus). 71 well-nourished children between 4 and 45 months of age were studied. After oral rehydration, the patients were randomly assigned to receive either Lactobacillus GG-fermented milk product 125 g (1010-11 CFU) twice daily (Group 1), Lactobacillus GG-freeze dried powder one dose (1010-11 CFU) twice daily (Group 2) or a placebo milk product (Group 3) with no lactic acid bacteria 125 g twice daily, each for 5 days. In addition, normal full diet otherwise free of fermented dairy products was given. Intestinal permeability was assessed by urinary recovery ratios of orally-administered lactulose (4g) and mannitol (0.8 g). The mean (SD) duration of diarrhoea after commencing the therapy was significantly shorter in Group 1, 1.4 (0.8) days, and in Group 2, 1.4 (0.8) days, than in Group 3, 2.4 (1.1) days; F= 8.70, p < 0.001. The decrease in the number of diarrhoeal stools became apparent after the first day of therapy in Group 1 and Group 2. After rehydration, each dietary group maintained a positive weight trend. The urinary lactulose/manniiol recovery ratios (means [95 % confidence intervals]) were on admission in Group 1, 0.09 [0.03,0.24]; Group 2, 0.12 [0.07, 0.22]; Group 3, 0.08 [0.04,0.18], and no significant alterations were observed at retesiing after 2 days of realimentation. The result indicates that early nutritional repletion after rehydration causes no mucosal disruption, and is beneficial for the recovery from diarrhoea. We suggest that fermented dairy products, with viable lactic acid bacteria that are able to colonize the gut, are effective in shortening the course of acute diarrhoea.