One hundred five Iranian subjects, ranging in age from 4 months to 25 years, were tested for lactose absorption and tolerance. After ingesting a lactose dose, on the basis of low blood glucose response, 68% of the subjects were malabsorbers. Prevalence of lactose malabsorption increased with age, i.e., 31% of the children less than 3 years of age malabsorbed lactose, whereas 86% of adults did so. Clinical manifestations of lactose intolerance were shown by 39% of all subjects; of those who were lactose malabsorbers, only 57% manifested clinical symptoms. Among the lactose malabsorbers, the lowest prevalence of clinical symptoms occurred among the children less than 3 years of age and the highest in adults.