Solitary median maxillary central incisor syndrome (SMMCI) syndrome is a unique developmental abnormality arising from an unknown event occurring between the 35th and 38th days in utero, and involving mieline structure of the head including the cranial bones, the maxilla and its container dentition (specifically the central incisor tooth germ), together with other midline structures of the body. The SMMCI tooth may be possibly occur as an isolated trait or in association with many other midline developmental anomalies. It is estimated to occur in 1:50000 live births. There is a wide variability in the phenotypic spectrum. SMMCI is considered one of the most minimal expressions of the holoprosencephaly spectrum. Mutation in the Sonic Hedgehog homolog (SHH) gene may be associated with SSMMCI, but recent studies suggests the existence of several other candidate genes. We described two patients with SMMCI. They presented a solitary median maxillary incisor, short stature, hipotelorism and corpus callosus anomalies found on magnetic resonance imaging (MRI). They also present severe hiponatremia. At the best of our knowledge, this is the first report of cases of SMMCI with hiponatremia. We suggest that the sodium disorder may be secondary to syndrome of inappropriate secretion of antidiuretic hormone (SIADH).