Intake and coefficient of absorption of nutrients were measured in 72 children during acute diarrhoea and 2 weeks after recovery. No diarrhoeal pathogens could be identified in 18 (25%) children (group I). Aetiology of diarrhoea was identified in rest of the 54 children (group II). Absorption of calorie, fat and carbohydrate during the recovery stage were similar in all 72 children. In group I, absorption of nutrients improved from acute stage to recovery stage as follows: fat from 62 to 85%, calories from 68 to 86% and carbohydrate from 81 to 91%. Absorption of nitrogen during the acute stage was significantly lower (P < 0.01) in group I (mean +/- SD: 2% +/- 56) than in group II (mean +/- SD: 49% +/- 28). The anion gap in the stool of group I children (32) was similar to those with shigellae (37) and rotavirus (38). This could partially be explained by the possible loss of anionic proteins, fatty acids and/or lactic acids in the diarrhoeal stool. Results suggest that the diarrhoea due to unknown aetiology is possibly of the invasive type. Further investigation is necessary to define the mechanism of nitrogen loss in acute diarrhoea of unknown aetiology.