We describe a child with Down's syndrome who developed an insulin-dependent diabetes mellitus at the age of 8 years and hypothyroidism at the age of 17 years. Because of the well known tendency to autoimmune diseases of patients with Down's syndrome, an autoantibody screening was undertaken. Only a low titre for gastric parietal cell antibodies was repeatedly found, but a gastric biopsy did not reveal chronic atrophic gastritis. Thyroid function should be checked periodically in patients with Down's syndrome since they might suffer from hypothyroidism which may not be recognized for a long time because of its latent onset.