In a controlled trial in Petrozavodsk, Karelia, the effects of oral rehydration and Lactobacillus strain GG (LGG) on recovery from acute diarrhoea (27% rotavirus, 21% bacterial aetiology) were studied in 123 children aged between 1 and 36 months of age. On admission to hospital, the patients were first randomized to receive either isotonic oral rehydration solution (ORS) with osmolarity 311 mosmol/l and sodium 90 mmol/l (WHO-ORS), or a hypotonic ORS with osmolarity 224 mosmol/l and sodium 60 mmol/l (Light-ORS), and thereafter randomized to receive either 5 x 10(9) colony forming units of LGG or a matching placebo. The two ORS performed equally for acute rehydration, and oral rehydration with either ORS was associated with a shorter duration of diarrhoea than intravenous rehydration (p = 0.036). Patients receiving LGG had a significantly shorter duration of watery diarrhoea [mean (SD) 2.7 (2.2) days] than those receiving the placebo [3.7 (2.8) days, p = 0.03]. LGG significantly shortened the duration of rotavirus diarrhoea but not diarrhoea with confirmed bacterial aetiology.