The main cause for unilateral non-formation or non-functioning of a kidney can be found in renal agenesis/aplasia and multicystic dysplastic kidney. Even though kidney donation at adult age is considered safe, studies in recent years have shown that this may be different in congenital solitary functioning kidneys. Whether this is based on dysplasia in the remaining kidney or based on glomerular hyperfiltration damage, follow-up has shown renal injury, defined as hypertension and/or proteinuria, in up to 32% of children with a congenital solitary functioning kidney. Therefore, long-term infrequent follow-up of all patients with a congenital solitary functioning kidney seems to be indicated.