Low body mass index (BMI, kg/m2) has been proposed as a practical measure of energy undernutrition although it has some well-known limitations. Some reports have suggested that those Australian Aborigines living a largely traditional way of life have low BMI without compromised health status and may have paradoxically high levels of subcutaneous adipose tissue. The evidence for low BMI in Australian Aborigines is reviewed from the mean data of 1,174 individuals in 26 groups of adults and from the individual data of 349 of these individuals, collected before 1970. Three of the nine groups of women had mean BMI less than 18.5 kg/m2 and 4% of the individual men and 14% of the individual women had values less than 16 kg/m2, a value regarded as indicating severe chronic energy deficiency. Skinfold thicknesses were greater than expected from the BMI, suggesting paradoxically high subcutaneous fatness. The contribution of long-leggedness to low BMI was estimated from the regression of BMI on the sitting height to stature ratio (SH/S). For the 26 groups, this was estimated to be 2 kg/m2, r2 = 31%. The relationship was weaker with the individual data, r2 = 15%. Body shape as evinced by low SH/S does contribute to low BMI in these Australian Aborigines. Single cut-offs of BMI are not applicable to all population groups and allowance may have to be made for body form when using BMI to assess nutritional status.