Indices for lean and fat mass adjusted for height derived from bioelectrical impedance for children aged 7 years have been published previously and their usefulness in the clinical assessment of undernutrition has been demonstrated. However, there is a need for norms that cover a wider age range and to explore their functional significance. The aim of the present study is to derive lean and fat indices for children aged 7–11 years and investigate associations with objective measures of cardio-respiratory fitness and grip strength. Subjects were 9574 children participating in the Avon Longitudinal Study of Parents and Children (ALSPAC). Bioelectrical impedance analysis (BIA) data collected longitudinally between ages 7 and 11 were used to derive lean and fat indices using the method of standardised residuals. Cardio-respiratory fitness (CRF) (9 years) and grip strength (11 years) were also measured. Correlation coefficients and 95 % CI were calculated to assess the strength of association between lean index, fat index and CRF and grip strength. Equations for calculating lean and fat indices in children aged 7–11 years relative to the ALSPAC population are presented. Lean index was linearly associated with CRF (rboys 0·20 (95 % CI 0·15, 0·25), rgirls 0·26 (95 % CI 0·22, 0·30)) and grip strength (rboys 0·29 (95 % CI 0·26, 0·32), rgirls 0·26 (95 % CI 0·23, 0·29)). BMI showed slightly weaker associations, while fat index was unrelated to either CRF or grip strength. Lean indices relate to muscle function and fitness while fat index does not.