We describe two patients who suffered transient neonatal diabetes mellitus (TDNM), due to paternal isodisomy of chromosome 6. One patient, now 5 years old, had severe intra-uterine growth retardation, but recovered normal growth parameters. The other patient, currently 12 years old, had a normal birth weight but showed impaired post-natal growth; in addition to TNDM the patient presented with cardiac and thyroid abnormalities. These cases may suggest that the clinical phenotype of TNDM is more variable than previously believed. The contribution of genetic and epigenetic factors needs to be determined to elucidate the phenotype-genotype relationships of this disease.