The genetic background of neonatally diagnosed diabetes has begun to become clear only in the last few years. Recognition of the disease mechanism has often allowed the replacement of insulin therapy by sulfonylurea, which has improved the patients' quality of life and diabetes control. We describe a male infant diagnosed with neonatal diabetes, and his mother, who has had a disease considered to be type 1 diabetes since infancy. Both were diagnosed with a mutation in the KCNJ11 gene coding for the beta cell potassium channel and causing neonatal diabetes. The patients were treated with sylfonylurea.