To determine the effect of a human Lactobacillus strain (Lactobacillus casei sp strain GG, Gefilac) on recovery from acute diarrhea (82% rotavirus), 71 well-nourished children between 4 and 45 months of age were studied. After oral rehydration, the patients randomly received either Lactobacillus GG-fermented milk product, 125 g (10(10-11) colony-forming units) twice daily (group 1); Lactobacillus GG freeze-dried powder, one dose (10(10-11) colony-forming units) twice daily (group 2); or a placebo, a pasteurized yogurt (group 3) 125 g twice daily; each diet was given for 5 days, in addition to normal full diet otherwise free of fermented dairy products. The mean (SD) duration of diarrhea 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 less than 0.001. After rehydration, each dietary group maintained a positive weight trend. The urinary lactulose-mannitol recovery ratios (means [95% confidence intervals]) on admission were 0.09 (0.03, 0.24) in group 1, 0.12 (0.07, 0.22) in group 2, and 0.08 (0.04, 0.18) in group 3; no significant alterations in intestinal permeability were observed at retesting after 2 days of realimentation. The result indicates that early nutritional repletion after rehydration causes no mucosal disruption and is beneficial for recovery from diarrhea. It is further suggested that Lactobacillus GG in the form of fermented milk or freeze-dried powder is effective in shortening the course of acute diarrhea.