Although recommended by some for diarrhoeal diseases, data that support treating children with gastroenteritis using probiotics are limited. Two randomized, placebo-controlled trials have now examined the efficacy of a probiotic to improve outcomes of gastroenteritis among preschool children. A total of 1,857 children aged 3–48 months who presented with gastroenteritis to paediatric departments in North America were enrolled in one of the two trials. They all received a 5-day course of a probiotic containing Lactobacillus rhamnosus or placebo, and the primary end point was moderate-to-severe symptoms of gastroenteritis within 14 days, defined using a clinical severity score. In both trials, children that received the probiotic did not have better outcomes than those who received placebo, and no significant differences were found between the groups for secondary outcomes, including duration of diarrhoea or vomiting.