BACKGROUND: In moderate acute malnutrition programs, it is common practice to not measure mid-upper arm circumference (MUAC) of children whose length is <67 cm. This is based on expert opinion that supplementation of shorter children with low MUAC and weight-for-height z score ≥-2 may increase risk of excessive fat accumulation. Our aim was to assess if shorter children gain more fat than taller children when treated for moderate acute malnutrition diagnosed by low MUAC alone. METHODS: In this observational study, we included children aged 6 to 23 months with a MUAC between 115 and 125 mm and a weight-for-height z score ≥-2. On the basis of length at admission, children were categorized as short if <67 cm and long if ≥67 cm. Linear mixed-effects models were used to assess body composition on the basis of deuterium dilution and skinfold thickness. RESULTS: After 12 weeks of supplementation, there was no difference in change in fat mass index (-0.038 kg/m2, 95% confidence interval [CI]: -0.257 to 0.181, P = .74) or fat-free mass index (0.061 kg/m2, 95% CI: -0.150 to 0.271, P = .57) in short versus long. In absolute terms, the short children gained both less fat-free mass (-230 g, 95% CI: -355 to -106, P < .001) and fat mass (-97 g, 95% CI: -205 to 10, P = .076). There was no difference in changes in absolute subscapular and triceps skinfold thickness and z scores (P > .5). CONCLUSIONS: Short children with low MUAC do not gain excessive fat during supplementation. With these data, we support a recommendation for policy change to include all children ≥6 months with low MUAC in supplementary feeding programs, regardless of length. The use of length as a criterion for measuring MUAC to determine treatment eligibility should be discontinued in policy and practice.