In this issue of Acta Paediatrica, Bekhof et al. report on the reliability of fluid balance charts for newborn infants (1), demonstrating that charts relate poorly to serial determinations of body weight. Of further concern, their results reveal no systematic deviation, that is, the charts seem both to overand underestimate the balance in a non-predictable manner. From their data, one cannot conclude whether the deviations are due mainly to errors in the recording of inor output of fluid, or both. The results are in accordance with studies in adults (2). The authors should be commended for their effort to elucidate the performance of one of the most common monitoring methods used in the neonatal intensive care unit. Perturbations of fluid and electrolyte homeostasis are commonly encountered in the ill and ⁄or preterm neonate. Most disturbances are primarily related to imbalances in the amount of water ⁄fluid provided relative to the losses, with deviations in electrolyte concentrations occurring concomitantly. Didactic examples are the rapid weight loss and hypernatraemia often observed in extremely preterm infants during the first days because of their high insensible water losses, and the overhydration and hyponatraemia commonly encountered in asphyxiated infants with transient oliguric renal failure. Needless to say, recording urinary output and keeping track of the fluid volumes provided has an important role in the monitoring of the intensive care patient, but the data do not provide reliable information on net fluid balance and might indeed be misleading. The maintenance of an adequate fluid balance has a tremendous potential impact on the shortand long-term outcome in babies receiving neonatal intensive care. In many cases, fluid balance disturbances complicate the management of and delay recovery from the primary illness. Moreover, the fluid balance disturbance per se (e.g. hyperor hyponatraemia) has repeatedly been shown to impact on the infants’ future health. It is encouraging to note that, in the high-tech setting of modern neonatology, a low-tech and low-cost care routine receives attention, and most importantly, merits publication. In several studies in the field of fluid management, conclusions have been drawn by retrospectively relating important neonatal outcomes such as the prevalence of BPD and PDA to the amount of fluid (derived from charts) provided during the first postnatal days (3,4). These studies suggest that a careful fluid restriction might reduce the risk of BPD and ⁄or PDA. In perspective of the conclusions from the Bekhof’s study, such analyses may have been greatly influenced by (the lack of) fluid chart performance. So – if fluid charts are of limited value – what methodology should we pursue for further evaluation and development? Weighing is considered the gold standard and is invaluable, being simple, reasonably low-cost and in most situations reliable (5,6). However, monitoring weight alone does not provide information on fluid input vs. output or on the relative changes in hydration and body solids. In addition, several studies support the notion that an early postnatal reduction in body water (commonly referred to as the physiological contraction of the extracellular fluid space) may take place in spite of an early increase in body weight (7,8). Thus, by judging fluid balance solely from a gain in weight, the Invited Commentary for Asperen et al., Reliability of the fluid balance in neonates, pages 479–483. Acta Paediatrica ISSN 0803–5253