During 1880 patient-months of treatment with continuous subcutaneous insulin infusion in 101 patients with IDDM, 36 episodes of acute, severe loss of glycemic control, including 29 with significant ketoacidosis, occurred in 20 patients. Fifteen episodes were attributable to failure of insulin delivery to the patient while 13 were precipitated by infection. Insufficiently frequent blood glucose monitoring, failure by patients to detect mechanical and technical problems with infusion systems, failure to adhere to "sick day" regimens, and delay in seeking medical help all contributed to the progression of a number of episodes. Thirst, nausea, and vomiting were the common clinical manifestations of decompensation; and the degree of acidemia was often mild in relation to the degree of hyperglycemia. Response to conventional management was usually prompt.