As part of a prospective, longitudinal study that was carried out in Hanoi, Vietnam (as from 1981), mid-upper-arm circumference (MUAC) measurements, and the weight and height of children were taken from children for the first year of life. These measurements were recorded monthly, 12-36 months: 3-monthly, 36-72 months: 6-monthly and yearly for children beyond 72 months. The aim of this study was to observe development of MUAC of children on a longitudinal basis. Mid-upper-arm circumference increases by about 1 cm for boys and 1.5 cm for girls between 1 and 5 years. To recognise malnutrition, the cut-off level of MUAC 13.5 cm gave high values for sensitivity and specificity only for children aged 6-12 months. The cut-off levels of 14.0, 14.5 and 15.0 cm were more appropriate for children aged 13-24, 25-36 and 37-60 months. These results suggest that a single cut-off point of MUAC 13.5 cm cannot be used in screening moderate malnutrition for all children under age 5 but should be elevated with the increasing age of children.