To explore the possibility for a statistically appropriate value of mid-upper arm circumference (MUAC) to identify the state of severe undernutrition, based on very low BMI, among adult Indian slum dwellers. Cross-sectional study on adults. Height and MUAC were recorded and BMI was computed. Chronic energy deficiency (CED) was determined using the WHO international guidelines as BMI<18·5 kg/m2 and normal as BMI≥18·5 kg/m2. Besides calculating mean, sd and 25th, 50th and 75th percentile values, multiple linear regression analysis was undertaken to assess the associations between age, MUAC and BMI. Receiver-operating characteristic curve analysis was performed to determine the best MUAC cut-off to identify CED status. The χ 2 test was used to assess significance of the difference in CED prevalence across MUAC categories. An urban slum in Midnapore town, West Bengal State, India. Male (n 467) and female (n 488) Indian slum dwellers. MUAC of 22·7 and 21·9 cm, respectively, in males and females were the best cut-off points to differentiate CED from non-CED. Results supported the validity of the WHO-recommended MUAC cut-offs for adults. There is still a need to establish statistically appropriate MUAC cut-offs to predict undernutrition and morbidity.