Salient aspects of prolonged metabolic studies on seven excessively labile diabetic patients and a review of the literature concerning causation and therapy of brittle diabetes are presented. Brittleness is redefined as "a syndrome of excessive insulin-sensitivity and ketosis-proneness manifested by extreme and unexplainable short-term and long-term fluctuations in the parameters of the disease". Evidence on the causation of hyperlability points to dysfunction of plasma-protein transport and of hepatic and peripheral tissue metabolism of insulin. No objectively demonstrable complete and lasting stabilization was possible by means of any antidiabetic or adjunctive therapeutic measures. However, achievement of quantitative improvement in the accuracy of regulation of diabetes and moderation in deviations from the acceptable range of parameters were feasible. To this end, therapy recommended for everyday use incorporates the following principles found to be most helpful in following the oscillations of the disease on the research ward: flexibility in the plan of therapy; accuracy, especially in timing of therapeutic events; and employment of an insulin program best suited to the patient's needs and comfort.