To determine whether depletion of liver glycogen or accumulation of liver fat (steatosis) was associated with the development of hypoglycaemia in children with fatal diarrhoeal illnesses, a case-control study was carried out comparing 17 children who had blood sugars less than or equal to 30 mg/dl with 17 age matched control children who had blood sugars greater than or equal to 59 mg/dl. The most common causes of diarrhoea in the hypoglycaemic children were Shigella sp. and Vibrio cholerae. The mean duration of diarrhoea before admission for the hypoglycaemic children, 7.8 d, was shorter than the 20.7 d for the controls (P less than 0.01). Most children in both groups showed signs of malnutrition, metabolic acidosis, and pneumonia. Liver specimens were obtained at post-mortem examination and stained with haematoxylin and eosin for general assessment and with periodic acid-Schiff stain for glycogen. Glycogen depletion was detected in 9 hypoglycaemic children and in only 3 control children (P less than 0.05). Hepatic steatosis, on the other hand, occurred with equal frequency in both groups but was associated with severe malnutrition in the hypoglycaemic patients (P less than 0.05). This result suggested that hypoglycaemia develops during acute diarrhoeal illnesses because gluconeogenesis fails to maintain the blood sugar concentration after depletion of liver glycogen. Frequent feeding of children with diarrhoea might help to prevent this complication.