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 ≤ 5 years old and 65% in older patients. As would be expected, 
patients with autism at age 5 > years demonstrated significant decline in 
lactase activity (24%, p = .02) in comparison with ≤ 5 years old autistic 
patients. Boys ≤ 5 years old with autism had 1.7 fold lower lactase activity 
than girls with autism (p = .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.