Abstract The aim of the study was to determine if a decrease in serum insulin-like growth factor I (Igf-I) levels under marginal malnutrition is responsible for the lower physical performance of girls of a low socio-economic status (LSES). Girls were selected after physical examination (Tanners stage 1) and anthropometric measurements (height, body mass or mb, body mass index or BMI=mb height2). Lean body mass mb,l was measured after skinfold thickness determination; serum IGF-I, by radioimmunoassay; maximal O2 consumption, (VO2max), directly during incremental exercise up to exhaustion; and maximal aerobic power (Wmax), using the force-velocity test. LSES girls (n=31) had been malnourished in the past and, currently, were suffering from marginal malnutrition: they were smaller (135.25.5 vs 146.14.3 cm), lighter (31.73.9 vs 37.65.0 kg), exhibited a lower mb,l (24.22.5 vs 27.53.0 kg) but same BMI compared with HSES (high socio-economic status) girls (n=32). Igf-I levels (27.77.9 vs 34.16.5 nmoll1), VO2max (45.26 4.72 vs 50.746.02 mlmin1kg1 LBM) and Wmax (6.001.15 vs 8.701.53 Wkg1mb,l were lower in LSES girls. Moreover, the differences in every parameter were not the consequence of the younger age (10.80.9 vs 11.20.6 years) of the LSES girls. Our results provide evidence that the lower Wmax of undernourished prepubertal girls was partly the consequence of alterations in muscle function at the qualitative level, as a result of a decrease in Igf-I levels. Conversely, under normal nutritional conditions, anthropometric characteristics only are explicatory factors for physical performances.