Thirty-five cases of renal acidosis in infants are presented. Clinically there is failure to thrive with vomiting and obstinate constipation starting from the fourth to sixth month of life. Biochemically there is a hyperchloraemic acidosis with a neutral or alkaline urine. Radiological evidence of renal calcification was present in 13 of the 35 cases. The treatment is to give sufficient alkali (as citrate, lactate or bicarbonate) to bring the alkali reserve up to the lower limit of normal. The prognosis is good, cure resulting in all cases. The etiology and pathology has been considered; it is suggested that the cause lies in an imbalance in the normal postnatal development of renal function.