A causal relation between infection with mumps virus and pancreatic endocrine and exocrine dysfunction is suggested by the occurrence of severe diabetic ketoacidosis and serum pancreatic amylase elevation in a patient with a significantly elevated mumps antigen titer. Sequential studies of beta cell secretion in the presence of exogenous insulin and insulin antibodies was accomplished by monitoring levels of C-peptide immunoreactivity. Secretion of proinsulin, C-peptide, and, presumably, insulin resumed after the acute episode and was associated with complete recovery of carbohydrate tolerance and cessation of insulin therapy. Serum amylase levels were persistently elevated, and a macroamylase was shown in the serum. However, the amylase released from the macroamylase complex by acidification presented different patterns at various stages of the patient's illness. Pancreatic-type amylase predominated during the acute phase, whereas the distribution of pancreatic-type and salivary-type amylase was essentially normal in samples taken several months later.