In his letter to the Editor Robert C. Tibbs, criticizes our suggestion that inadequate information has been published to establish that phenformin is a beneficial adjunct to insulin therapy of diabetes mellitus in children. He cites his personal experience which confirms the impressions of several authorsthat combined insulin-biguanide therapy plus the increased supervision such treatment entails has been associated with lowering of insulin dosage, stabilization of blood sugar, and reduction of the tendency to acidosis in a few "brittle" diabetic patients.