Background: Middle-aged and older American men and women have almost twice the rate of diabetes of men and women in England. This differential was not explained by conventional risk factors including age, smoking, social position and body mass index (BMI). Methods: This study used large and representative samples of non-minority adults aged 52-85 years taken from the 1999-2006 American National Health and Nutrition Examination Survey (NHANES) and the 2004 English Longitudinal Study of Aging. The surveys contain self-reported and objective biological disease markers of diabetes as well as indicators of major risk factors for diabetes including anthropometric measures of BMI, height and waist circumference. Results: The older American population has much higher rates of diabetes than the English population-a differential not yet explained, but this population also has higher waist circumference at each level of BMI than does the equivalent group in England. By controlling for such waist circumference differences and allowing for different effects of waist on diabetes in each country, approximately three-quarters of the country differences for women and 38% among men can be explained. Conclusions: Higher rates of diabetes in the US old-age population than in England were largely accounted for by raised waist circumference and not BMI differences, especially among women. In addition, elevated diabetes risk associated with higher waist circumference in the USA as opposed to England could arise as a result of a number of different mechanisms. Investigation of the relative importance of such mechanisms is an important topic for further study.