Glucose tolerance, plasma insulin and growth hormone response to a glucose load were studied in 10 patients with Turner's syndrome and 3 patients with pure gonadal dysgenesis. It was found that 60 per cent of the patients with Turner's syndrome had a diabetic GTT. This is a frequency which is significantly higher than expected. None of the patients with pure gonadal dysgenesis had a diabetic GTT. The diabetes in patients with Turner's syndrome was of a mild type as is most frequently found in maturity-onset diabetes. The insulin response was, however, different from what is usually found in maturity-onset diabetes. The patients with Turner's syndrome showed a brisk rise and prolonged high levels of plasma insulin after glucose ingestion. An early plasma growth hormone peak was found in patients with Turner's syndrome. The significance of this finding for the pathogenesis of mild diabetes in patients with Turner's syndrome is discussed together with other possible aetiologic and pathogenetic factors.