We determined the frequency of acute complications associated with insulin pump therapy in 161 insulin-dependent patients followed up for a total of 2,978 patient-months. Diabetes control improved substantively with pump therapy, but 42% of the patients experienced one or more acute complications while using insulin pumps. Infected infusion sites, ketoacidosis, and hypoglycemic coma occurred once in every 27, 78, and 175 patient-months, respectively. More patients experienced ketoacidosis after the onset of pump therapy than in an equivalent interval immediately before the onset of pump therapy. Ketoacidosis also occurred in more patients using pump therapy than in a comparison group of 165 patients receiving conventional insulin injections surveyed during an equivalent period. The frequency of hypoglycemic coma was not significantly changed by pump therapy.