The aim of this study was to determine if supplementation of healthy children with milk fermented by yogurt cultures and Lactobacillus casei strain DN-114 001 could affect the incidence of acute diarrhoea when compared with traditional yogurt. The study was a multicentre, randomised, double-blind trial, conducted over four months, on 928 children aged, at inclusion, 6-24 months. The study consisted of two periods: supplementation and observation. Subjects were supplemented daily with 100 g of one of the two dairy products being tested: standard yogurt and milk fermented by yogurt cultures and Lactobacillus casei (10(8) cfu/ml). Frequency or duration of any diarrhoea episode was evaluated. As far as frequency was concerned there was a statistically significant difference between the groups, the incidence of diarrhoea being significantly reduced by supplementation with L. casei fermented milk (15.9%) compared with yogurt (22%) (p = 0.03). These results suggest an additional benefit of L. casei in acute diarrhoea in children compared with standard yogurt.