Objectives: To investigate the use of armspan as a proxy for height in the assessment of nutritional status using body mass index (BMI) for four ethnic groups in Ethiopia. Design: Cross-sectional study. Setting: Four regions in Ethiopia, namely Oromia, Amhara, Tigre and Somali Region. Subjects: A total of 1706 (884 males and 822 females) Ethiopians aged 18 -50 y from four different ethnic groups. Methods: Anthropometric measurements (weight, height and armspan) were obtained using standard techniques. BMI using height (BMI-ht) and using armspan (BMI-as) were calculated, t-tests were used to compare means, and linear regression to investigate the relationship between BMI-ht and BMI-as. Results: Ethnic and sex differences in the relationship between height and armspan, and their derived variables (BMI-ht and BMI-as), were found. Armspan and height (r ¼ 0.83 -0.9), and BMI-ht and BMI-as (r ¼ 0.89 -0.97), were highly correlated in all ethnic groups. BMI-as cut-offs equivalent to the conventional BMI-ht classification of chronic energy deficiency were similar in the Oromo, Amhara and Tigre, but substantially higher in the Somalis. Conclusion: Armspan can be used as a proxy for height to estimate BMI, but the relationship between the two measures varies considerably with ethnicity and sex. Unless sex-and ethnicity-specific cut-offs are applied, the use of BMI-as using conventional cut-offs will overestimate the prevalence of underweight in these populations.