BACKGROUND Lactose intolerance resulting in gastrointestinal symptoms is a common health concern. Diagnosis and management of this condition remain unclear. OBJECTIVE To assess the maximum tolerable dose of lactose and interventions for reducing symptoms of lactose intolerance among persons with lactose intolerance and malabsorption. METHODS Multiple electronic databases, including MEDLINE and the Cochrane Library, for trials published in English from 1967 through November 2009. METHODS Randomized, controlled trials of individuals with lactose intolerance or malabsorption. METHODS Three investigators independently reviewed articles, extracted data, and assessed study quality. RESULTS 36 unique randomized studies (26 on lactase- or lactose-hydrolyzed milk supplements, lactose-reduced milk, or tolerable doses of lactose; 7 on probiotics; 2 on incremental lactose administration for colonic adaptation; and 1 on another agent) met inclusion criteria. Moderate-quality evidence indicated that 12 to 15 g of lactose (approximately 1 cup of milk) is well tolerated by most adults. Evidence was insufficient that lactose-reduced solution or milk with a lactose content of 0 to 2 g, compared with greater than 12 g, is effective in reducing symptoms of lactose intolerance. Evidence for probiotics, colonic adaptation, and other agents was also insufficient. CONCLUSIONS Most studies evaluated persons with lactose malabsorption rather than lactose intolerance. Variation in enrollment criteria, outcome reporting, and the composition and dosing of studied agents precluded pooling of results and limited interpretation. CONCLUSIONS Most individuals with presumed lactose intolerance or malabsorption can tolerate 12 to 15 g of lactose. Additional studies are needed to determine the effectiveness of lactose intolerance treatment.