Diarrhea 3, secretory sodium, congenital, with or without other congenital anomalies (DIAR3) is a rare autosomal disorder, characterized by severe watery diarrhea containing high concentrations of sodium, hyponatremia and metabolic acidosis. It is caused by a functional defect in one of the sodium/proton exchangers localized to the apical membrane of small intestinal epithelial cells. Diagnosis of DIAR3 is based on the findings of a life-threatening secretory diarrhea, severe metabolic acidosis, and hyponatremia secondary to extraordinarily high fecal losses of sodium, with low or normal excretion of urinary sodium. So far, less than 15 patients have been described worldwide.