Plasma glucose concentration is the best predictor for the development of non-insulin-dependent diabetes mellitus (NIDDM). However, obesity is also a recognized risk factor for development of the disease, and is easier to track over time. Thus obesity could be of considerable clinical importance as a predictor of diabetes. Studies have shown that the degree of overweight, the change in weight and the duration of overweight are all separate predictors of diabetes. The British Regional Heart Study showed that an increasing body mass index (BMI) was associated with increased risk of developing diabetes, even at BMI values not considered obese. A separate study showed that weight gain increased the risk of diabetes independently of BMI, while weight loss decreased the risk. The duration of obesity was also an important factor in developing NIDDM. A long duration increased the risk of diabetes, irrespective of the final BMI value. The effects of obesity on insulin action have also been investigated. Studies have shown that insulin sensitivity is inversely related to insulin secretion, with a disproportionate increase in insulin secretion seen with decreasing sensitivity. A recent European study showed that the prevalence of both insulin hypersecretion and insulin resistance increased with increasing BMI. Thus, in obesity, higher insulin levels are necessary to maintain glucose tolerance, leading to increased stress on the beta-cells. In obese individuals, weight loss improved insulin sensitivity in proportion to the degree of weight loss, leading to decreased insulin secretion. Weight loss can therefore, at least in the short term, act to decrease the risk of developing diabetes by reducing insulin resistance, and thus relieving beta-cell stress, the factor ultimately responsible for hyperglycaemia in predisposed individuals.