Clinical findings in 21 Finnish children with congenital chloride diarrhoea are reported. Inheritance of this disease by the autosomal recessive mode is established. All children were born 1-8 weeks prematurely. Hydramnios was present in every case and no meconium was observed; intrauterine onset of diarrhoea is thus apparent. In most cases the diarrhoea or passing of large volumes of "urine" was noted on the first day of life and the abdomen was usually large and distended. The neonatla weight loss was abnormally large, and was associated with hypochloraemia and hyponatraemia. Some infants survived the neonatal period without adequate therapy. They presented later with failure to thrive and usually had hypochloraemia, hypokalaemia, and metabolic alkalosis associated with hyperaldosteronism. However, these features may be absent and the diagnosis is based on a history of hydramnios and diarrhoea, and a faecal Cl- concentration which always exceeds 90 mmol/l when fluid and electrolyte deficits have been corrected. Lower faecal Cl- concentrations were seen only in chronic hypochloraemia, which is also associated with achloriduria. Adequate treatment consists of full continuous replacement of the faecal losses of water, NaCl, and KCl. This should be given intravenously in the early neonatal period; later a solution can be taken orally with meals. The dose has to be adjusted to maintain normal serum electrolyte concentrations, normal blood pH, and some chloriduria. This therapy prevents the renal lesions and the retarded growth and psychomotor development which were seen in the children who were diagnosed late and in those who received inadequate replacement therapy. The watery diarrhoea persists and increases slightly with age, though patients learn to live with their disease and to make an adequate social adjustment.