The condition of "megacystis-microcolon" is a very rare condition which can now be diagnosed ante-natally due to ultrasound. The presence of a megacystis (large bladder) with increased levels of amniotic fluid makes it possible to eliminate other ante-natal causes of dilatation of the bladder and in particular when it is found in a female fetus (as it is in 80% of cases). At birth, one finds, apart from the massive dilatation of the bladder with very little effect on the upper urinary tract, a microcolon with complete absence of peristalsis. This last abnormality always leads to a catastrophic short term result because treatment is quite ineffective. Many histological studies have been carried out and these sometimes differ from one another but it is possible to think that the condition is due to pathology of neurogenic origin in the muscle of the bladder and the aetiology of this is unknown. Finally, because there have been a few familial cases, there may be a genetic element in this illness that has such a poor prognosis.