Researchers in the USA randomly assigned 132 severely obese subjects with a mean body-mass index of 43 and a high prevalence of diabetes (39%) or the metabolic syndrome (43%) to the carbohydrate-restricted (Atkins) diet or a calorie- and fat-restricted (low-fat) diet. A total of 79 subjects completed the 6-month study. An analysis including all subjects, with the last observation carried forward for those that dropped out, showed that subjects on the low-carbohydrate diet lost more weight than those on the low-fat diet and had greater decreases in triglyceride levels, irrespective of the use or non-use of hypoglycaemic or lipid-lowering medications. Insulin sensitivity, measured only in subjects without diabetes, also improved more among subjects on the low-carbohydrate diet. The amount of weight lost and assignment to the low-carbohydrate diet were independent predictors of improvement in triglyceride levels and insulin sensitivity. Severely obese subjects with a high prevalence of diabetes or the metabolic syndrome lost more weight during 6 months on a carbohydrate-restricted diet than on a calorie- and fat-restricted diet, with a relative improvement in insulin sensitivity and triglyceride levels, even after adjustment for the amount of weight lost. The authors caution that these findings should be interpreted with care, given the small magnitude of overall and between-group differences in weight loss in these markedly obese subjects and the short duration of the study.