Comparisons are made between the incidence, prognosis and treatment of juvenile-onset diabetes and other endocrinopathies in the young. 548 patients with insulin deficient diabetes diagnosed before 20 years of age have been reviewed. Excess mortality, especially at 35--40 years of age was found. Profiles of blood glucose and serum insulin have been studied and compared to those of normal subjects. The variation of insulin absorption and effect of insulin antibodies on the free insulin levels achieved after exogenous insulin injections have been demonstrated. The common occurrence of nocturnal subclinical hypoglycaemia following intermediate or long-acting insulin was often found to be the cause of poor diabetic control. Five out of 33 patients with 'difficult' diabetes had an unexplained resistance to high levels of free-insulin. The value of self-monitoring and HbAl measurements in the improvement of diabetic control and possibly life expectation is reviewed. The incidence of thyroid disease was found to be increased in 1779 insulin deficient diabetics of all ages and persistence of islet-cell antibodies suggests that the diabetes may be due to autoimmunity in some of these patients.