Paediatric registries worldwide report that congenital abnormalities of the kidney and urinary tract (CAKUT) account for approximately 50% of end-stage renal failure and other congenital and familial diseases account for another 20% (together 70%). Does the same hold true for young adults? Almost nothing has been published about primary renal disease in adults who have reached end-stage before 30 years of age. I have reviewed the UK renal registry (2000-2006) and the United States Renal Data System (USRDS) data base (2005) to answer this question. While paediatric registries have reduced the number of children with 'no specific diagnosis' from 39% in 1976 to fewer than 5%, the adult registries still report rates of 20-27%, which rise to 28-36% when all unspecified groups, predominantly 'glomerulonephritis (GN) (histologically not examined)', are considered together. For UK data, this rise in 'no specific diagnosis' mirrors a fall in CAKUT to 26% for the age group 18-21 years. According to USRDS data, CAKUT falls from 31% for ages 0-19 years to only 5% for ages 20-30 years. Nephrologists probably under-diagnose CAKUT in young adult patients, and this diagnosis can account for many of the 30% that currently have no specified primary renal disease.