To obtain information on the safety and efficacy of the gluten-free/casein-free (GFCF) diet, we placed 14 children with autism, age 3–5 years, on the diet for 4–6 weeks and then conducted a double-blind, placebo-controlled challenge study for 12 weeks while continuing the diet, with a 12-week follow-up. Dietary challenges were delivered via weekly snacks that contained gluten, casein, gluten and casein, or placebo. With nutritional counseling, the diet was safe and well-tolerated.