To examine the effects of a digestive enzyme supplement in improving expressive language, behaviour and other symptoms in children with Autism Spectrum Disorder. Randomized, double-blind placebo-controlled trial using crossover design over 6 months for 43 children, aged 3–8 years. Outcome measurement tools included monthly Global Behaviour Rating Scales, Additional Rating Scales of other symptoms by parents and therapists, and monthly completion of the Rescorla Language Development Survey. Compared with placebo, treatment with enzyme was not associated with clinically significant improvement in behaviour, food variety, gastrointestinal symptoms, sleep quality, engagement with therapist, or the Language Development Survey Vocabulary or Sentence Complexity Scores. A small statistically significant improvement on enzyme therapy was seen for the food variety scores. No clinically significant effect improvement of autism symptoms with enzyme use was shown with this trial, however, possible effects on improvement in food variety warrants further detailed investigation.