A retrospective, case-control study of predictors of moderate to severe dehydration in patients seen at the Diarrhoeal Disease Control Training Centre, Addis Abeba, Ethiopia, over four years (1986-1989) is presented. A total of 1626 cases with moderate to severe dehydration and an equal number of admission year matched, randomly selected, controls with no clinical evidence of dehydration were analysed. Among the possible predictors considered watery diarrhoea, duration of illness of more than 3 days, high stooling rate, frequent vomiting, fever, early weaning and antecedent protein energy malnutrition (PEM) were found significant. An increase in severity of dehydration when diarrhoea occurred in association with systemic infections, most prominently in infancy, was documented. Use of these findings in the clinical management of patients and in health education of mothers and other care-takers is suggested. Reevaluation of these and other potential predictors by a carefully designed prospective study is proposed.