Weight reduction is one of the most effective therapies for obese non-insulin-dependent diabetes mellitus (NIDDM), but the success rate with conventional diets has been disappointing. The development of very-low-calorie diets (VLCDs) over the last two decades has provided an alternative approach to the treatment of uncomplicated obesity and is increasingly being used to treat obese NIDDM. This review focuses on the role of VLCDs in the treatment of obese NIDDM, the mechanisms underlying their efficacy, and the controversies surrounding their use. VLCDs provide 400-800 cal/day of high-quality protein and carbohydrate fortified with vitamins, minerals, and trace elements. Weight loss is initially very rapid, followed by steady reduction at a rate of 1-3 kg/wk. Metabolic benefits occur quickly with only modest weight reduction, suggesting that caloric restriction plays a more critical role. Multiple mechanisms account for improved glycemic control, including reduced hepatic glucose output, increased insulin action in the liver and peripheral tissues, and enhanced insulin secretion. VLCDs have the added benefit of rapid improvement in concomitant medical problems, particularly hypertension and hyperlipidemia, that could otherwise accelerate the development of some diabetic complications. Numerous controversies surround VLCD therapy, the most critical of which is its safety. However, recent studies indicate that VLCDs are safe for use by obese NIDDM patients in a medical setting closely supervised by an experienced physician. Contraindications to the diet, side effects, and recommended management are reviewed, as well as the role of adjunctive treatments, including behavioral modification and exercise. We present the perspective that, in most cases, the numerous metabolic benefits derived from VLCD therapy by the obese NIDDM patient outweigh its risks. Furthermore, recent data suggest that VLCD therapy may provide long-term benefits to the obese diabetic patient, despite weight regain.