Since the mechanisms underlying hyponatraemia in meningitis are poorly understood, we retrospectively reviewed the records of 187 paediatric patients with bacterial meningitis treated at the Department of Pediatrics, University of Bern, Switzerland, between 1982 and 1994. The degree of dehydration calculated from naked weight on admission and at 5 days was consistently (by 2.8 x 10(-2) and significantly more pronounced in 30 hyponatraemic (plasma sodium 130 mmol l-1 or less) than in 157 normonatraemic patients (plasma sodium 131 mmol l-1 or more). Furthermore, a tendency towards reduced sodium excretion was noted in hyponatraemic patients. The results suggest that in bacterial meningitis hyponatraemia is mostly induced by clinically latent fluid volume depletion.