Protein-energy malnutrition and obesity are the most common nutritional disorders that complicate the clinical course of children with neoplastic diseases. Sensitive measures of nutritional status should be used to detect these problems in children with cancer. Height and weight measurements are the mainstay of the nutritional assessment of the child. These measurements can be converted to growth velocities or to height-for-age and weight-for-height Z-scores or percent of expected values to provide a measure of the degree of under- or over-nutrition in the child. Skinfold thickness and circumference measurements of the arms, legs and/or trunk may be useful to characterize the changes in peripheral fat depots and muscle mass, respectively. However, the assessments of body composition using these measurements are subject to methodological error because selected skinfold sites are excluded. Whole-body potassium, measured by 40K counting, and total body water, measured by deuterium or 18O dilution, serve as "gold standards" to determine the lean body mass and body fat status of the child, but these techniques may not be practical in all settings. The assessment of the nutritional status of the child serves as a guide to early nutritional intervention. Indicators for early nutritional intervention include: (1) height-for-age and weight-for-height or -age Z-scores more than 2 SD below the mean for age, (2) height-for-age measurements less than 95% of expected, (3) weight-for-height measurements less than 90% or greater than 120% of expected and (4) height velocities less than 5 cm/year after 2 years of age. Early nutritional intervention is essential to restore normal body composition, reverse linear growth arrest, promote tolerance to chemotherapeutic and radiation regimens and improve the quality of life in children with cancer.