This study of 200 Uruguayans between 0 and 86 years old was designed to determine the prevalence of lactose malabsorption. Lactose intolerance is defined as a clinical syndrome of abdominal pain, diarrhea, flatulence, and bloating after the ingestion of a standard lactose tolerance test dose (2 g of lactose per kilogram of body weight or 50 g/m2 of body surface area, maximum 50 g in a 20% water solution). Lactose malabsorption refers to the state in which dietary lactose remains unhydrolyzed and subsequently unabsorbed from the gastrointestinal tract; symptoms may or may not result from lactose malabsorption. The technique of breath hydrogen (H2) was used after ingestion of 2 g/kg body weight to a maximum of 50 g in a 20% solution. There was no lactose malabsorption in children younger than 5 years old. The prevalence increases progressively after the age of 5, and in adolescence the percentage of malabsorption is similar to that in adults, who show 65% lactose malabsorption, with 25% asymptomatic and 40% intolerant. In 109 white adults, the prevalence of lactose malabsorption is 63%, with 24% asymptomatic and 39% intolerant. In 11 black adults, lactose malabsorption is 82%, with 27% asymptomatic and 55% intolerant. The difference between white and black adults is statistically significant (p less than 0.05). The H2 test is simple, reliable, noninvasive, and appropriate to study large populations.