The objective of this study was to determine if supplementation with milk fermented by yogurt cultures and Lactobacillus casei (strain DN-114 001) could lessen acute diarrhoea in healthy children. The study was conducted over six months, with 287 children aged 18.9 (SD 6.0) months, comprising three periods of one month supplementation, each month being followed by one month without supplementation. Subjects were supplemented daily with either 125 g or 250 g (according to age) of one of three tested dairy products: standard yogurt, milk fermented by yogurt cultures and Lactobacillus casei (10(8) cfu/ml), or a jellied milk (control product). A daily record was kept of the number and type of stools. Although the incidence of diarrhoea was not shown to be different between the groups, the severity of diarrhoea over the six-month study was significantly decreased (4.3 days) with the supplementation of L. casei fermented milk compared with the jellied milk (8.0 days) (p = 0.009).