Skeletal muscle development is one of the key features of childhood and adolescence. Determining maximal isometric grip force (MIGF) using a hand-held Jamar dynamometer is a simple method to quantify one aspect of muscle function. Presently available reference data present MIGF as a function of chronological age. However, muscle force is largely determined by body size, and many children undergoing muscle performance tests in the clinical setting suffer from growth retardation secondary to a chronic disorder. Reference data were established from simple regressions between age or log height and log MIGF in a population of 315 healthy children and adolescents aged 6 to 19 y (157 girls). These data were used to calculate age- or height-dependent SD scores (SDS) for MIGF in three pediatric patient groups. In renal graft recipients (n = 14), the age-dependent MIGF SDS was markedly decreased (-2.5 +/- 1.9; mean +/- SD). However, these patients had short stature (height SDS, -2.5 +/- 1.2), and the height-dependent MIGF SDS was close to normal (-0.4 +/- 1.5). Similarly, in cystic fibrosis patients (n = 13) age-dependent MIGF SDS was -1.6 +/- 1.6, but height-dependent MIGF SDS was -0.5 +/- 1.1. Children with epilepsy who were taking anticonvulsant therapy (n = 34) had normal stature, and consequently age- and height-dependent MIGF SDS were similar (0.4 +/- 1.0 and 0.4 +/- 0.8, respectively). In conclusion, MIGF determination provides information on an important aspect of physical development. Height should be taken into account to avoid misinterpretation.