Intestinal disaccharidase activities were measured in 199 individuals with autism to determine the frequency of enzyme deficiency. All patients had duodenal biopsies that were evaluated morphologically and assayed for lactase, sucrase, and maltase activity. Frequency of lactase deficiency was 58% in autistic children less than or equal to 5 years old and 65% in older patients. As would be expected, patients with autism at age 5 greater than years demonstrated significant decline in lactase activity (24%, p = 0.02) in comparison with less than or equal to 5 years old autistic patients. Boys less than or equal to 5 years old with autism had 1.7 fold lower lactase activity than girls with autism (p = 0.02). Only 6% of autistic patients had intestinal inflammation. Lactase deficiency not associated with intestinal inflammation or injury is common in autistic children and may contribute to abdominal discomfort, pain and observed aberrant behavior. Most autistic children with lactose intolerance are not identified by clinical history. (Contains 7 tables.)