Recent position statements from the ADA and other health professional organisations have recommended low carb diets for the management of diabetes. In addition a number of studies have suggested that low carb diets can result in weight loss with additional glycaemic and lipaemic benefits. The New Zealand Obese (NZO) mouse is a polygenic model of obesity that displays glucose intolerance, insulin resistance and defects in insulin secretion. To determine whether a low carb diet would be beneficial, NZO mice were fed either a chow high carb/low fat (77% carb, 20% protein 3% fat w/w) or low carb/high fat diet (20% carb, 20% protein, 60% fat w/w) for 8 weeks and glucose tolerance, insulin secretion and islet histology determined.  Mice on the low carb diet gained significantly more weight (23.6±2.8 vs 12.0±0.9 g, P<0.0005), displayed fasting hyperglycaemia (15.7±1.2 vs 9.2±0.7 mM, P<0.0005) and hyperinsulinaemia (1.8±0.6 vs 0.4±0.1 ng/ml, P<0.05), glucose intolerance (AUCglc 3147.9±165.1 vs 2385.9±218.0 mMx120min, P<0.01) and impaired insulin secretion (AUCins 23.4±12.8 vs 46.4±7.3 ng/mlx30min, P=0.07). Beta cell mass was not different (10.4±2.9 vs 7.3±0.8 mg), while serum total cholesterol was higher (1.4±0.1 vs1.2±0.1 mg/ml, P=0.07) and serum triglycerides were lower (0.8±0.1 vs 1.8±0.2 mg/ml, P<0.005) in low carb fed compared with chow fed NZO mice. We conclude that in the NZO mouse a low carb diet increased weight gain which was associated with worsening of glycaemic control.