Children suffering from congenital chloride diarrhoea (CCD) have an increased resistance to dental caries. The basic defect in this disease is impaired Cl transport in the distal intestine. Treatment consists of life-long oral replacement of the electrolytes and water lost in the diarrhoea Out of nine affected children (5.5–12.0 yr) four were caries-free, and the mean defs + DMFS count for the group was 4.1. According to recent Finnish statistics a mean of 27 affected tooth surfaces per child could be expected at these age levels. Most of the deciduous teeth showed enamel hypoplasia. Some salivary reactions deviated from normal conditions. At present the reason for the increased resistance to dental caries remains obscure.