Probiotics are widely used by patients with Crohn's disease (CD) in an attempt to improve their health, but few controlled studies have been done to evaluate the efficacy of these therapies. We conducted a randomized, placebo-controlled trial of the probiotic Lactobacillus rhamnosus strain GG (LGG) to see if the addition of LGG to standard therapy prolonged remission in children with CD. Concomitant medications allowed in the study included aminosalicylates, 6-mercaptopurine, azathioprine, and low-dose alternate day corticosteroids. Seventy-five children (age range, 5-21 yr) with CD in remission were randomized to either LGG (n = 39) or placebo (n = 36) and followed for up to 2 years. The median time to relapse was 9.8 months in the LGG group and 11.0 months in the placebo group (P = 0.24); 31% (12/39) of patients in the LGG group developed a relapse compared with 6/36 (17%) of the placebo group (P = 0.18). The LGG was well tolerated, with a side effect profile comparable with placebo. This study suggests that LGG does not prolong time to relapse in children with CD when given as an adjunct to standard therapy.