The purpose of the study was to compare the use of absolute cut off values of the mid-upper arm circumference (MUAC) with age- and sex-adjusted z scores of the MUAC in the identification of acute undernutrition (wasting) in children up to 60 months of age. In Nchelenge, northern Zambia, 275 children from the community, selected by a two-stage cluster sampling procedure, and 105 hospitalized children with protein energy malnutrition (PEM) individually matched for age, sex, village and under-five clinic attendance with 104 controls, were clinically and anthropometrically (weight, height, MUAC) examined. zScores for weight for height and MUAC were calculated and PEM was classified according to a modified Wellcome scheme. For community prevalence rates of wasting in various age groups, MUAC < or = -4 z scores more closely paralleled W/H < or = -2 z scores than MUAC <125 mm. To identify individual children with wasting, MUAC < or = -2 z scores gave a better sensitivity than MUAC <125 mm. In hospitalized PEM children, z scores appeared to offer no advantages over absolute MUAC values in identifying the presence of wasting. Applicability of the MUAC and optimal cut off values may differ according to the setting in which the MUAC is to be applied.