The effects of an episode of acute classical dengue fever on extracellular water (ECW), intracellular water (ICW), and total body water (TBW) were measured in nine patients using conventional dilution techniques; and the findings were compared with the outcome variables from whole body impedance spectroscopy (BIS), extracellular fluid resistance (Recf), and intracellular fluid resistance (Ricf). The patients were assessed on admission with febrile presentation (DI), at discharge after the defervescence of the fever cycle at about five days postadmission (DII), and seven days thereafter (DIII). As a reference group, 15 persons without acute or chronic illness were enrolled. Total body water was unaltered during the course of disease and was not different from that in normal healthy subjects. However, body water shifted from the intracellular to the extracellular compartment in patients from the acute phase to convalescence, as reflected in the ratios of ECW/TBW and ECW/ICW. These ratios were significantly higher in convalescent dengue patients (DIII) than in the reference group (p < 0.05). Increasing ECW, from the acute phase of the disease to convalescence, was associated with a significant decrease in Recf (719 +/- 95, 693 +/- 89, 643 +/- 81 omega; p < 0.0001) and in Recf/Ricf (p < 0.01). Recf and Recf/Ricf were higher in the acute phase (DI) of dengue fever compared to controls (p < 0.05). We conclude that dengue fever is characterized by a relative expansion of ECW during the course of disease and convalescence. BIS was sensitive in determining the hydrational profile in dengue fever patients.