The hospital admission and mortality rates of patients with diabetic emergencies, such as diabetic ketoacidosis (DKA) and hyperglycemic hyperosmolar nonketotic syndrome (HHNS), are higher in black patients than in white patients with diabetes. However, there is limited data describing the precipitating events and response to treatment in black patients. Analysis of their clinical characteristics and response to medical therapy is needed to evaluate the impact of programs designed to reduce the development of these acute metabolic complications. A prospective evaluation was conducted of 144 consecutive patients with DKA and 23 patients with HHNS admitted to a large inner-city hospital between July 1993 and October 1994. In patients previously diagnosed as having diabetes, poor compliance with insulin therapy was the major precipitating cause for DKA (49%) and HHNS (42%). Alcohol or cocaine abuse was a contributing factor for noncompliance and was present in 35% and 13% of patients with DKA and in 44% and 9% of patients with HHNS, respectively. Newly diagnosed diabetes accounted for 17% of patients with DKA and HHNS. Obesity (body mass index > 28 kg/m2 [the weight in kilograms divided by the square of the height in meters]) was present in 29% of patients with DKA and in 17% with HHNS and was most common in patients with DKA who were newly diagnosed as having diabetes (56%). Patients were treated by residents, who used a low-dose insulin protocol with an algorithm for insulin adjustment in 88 of 144 patients with DKA and 14 of 23 patients with HHNS. Although there was no difference in mortality rates or time needed to correct hyperglycemia or ketoacidosis, the use of the protocol significantly reduced the risk of hypoglycemia (5%) compared with patients treated without a protocol (23%) (P < .01). In urban black patients, poor compliance with insulin therapy was the main precipitating cause of acute metabolic decompensation, and substance abuse was a significant contributing factor for noncompliance. Obesity is common in black patients with DKA; it was present in more than half of those with newly diagnosed diabetes. Improved patient education and better access to medical care might reduce the development of these hyperglycemic emergencies.