To identify the factors related to dehydration from diarrhoea, a hospital-based case-control study was carried out among under-2-year-old Bangladeshi children. The study compared 80 cases who had 'some' or severe dehydration with 160 age-matched controls who had 'no signs' of dehydration. All the cases and controls were examined and the mothers were interviewed in the hospital and followed at home on the 14th day of illness. Thirty-eight factors were studied for their probable influence on the development of dehydration. In bivariate analysis, 17 factors were found to be associated significantly with the development of dehydration and were treated with stepwise logistic regression analysis. A combination of vomiting, oral rehydration therapy at home, mother's dirty finger nails, and residing more than 3 km away from the hospital provided the maximum sensitivity (77.5 per cent) and specificity (91.2 per cent) for predicting development of dehydration. These prognostic factors would be helpful for community health workers to identify children at risk of developing diarrhoea-associated dehydration, and preventive strategies could be designed to alter the prognostic factors.