OBJECTIVES: Evaluate, using a markovian probabilistic model, the development of renal disease in Fabry Disease(FD), comparing the probability of developing the different stages of renal disease in those with FD using Enzyme replacement therapy (ERT) with those not using ERT.  METHODS: A model with pre-defined 3 possible stages was used for patients with FD and renal dysfunction. Progression to a different stage or having a stable disease were possible. Less severe ‘stages’ could only migrate to more severe ‘stages’. Three year time cycles were used. The probabilities ERT x without ERT were compared. RESULTS: Probabilities between the 2 arms: 1)The probability  of having proteinuria only and being stable in 3y was 56% in those treated versus 48% in those untreated; 2)Also for those with proteinuria only, with ERT the probability of progressing to renal impairment in 3y was reduced, to 24% compared to 32% in those untreated; and 3)For those receiving ERT already with renal impairment, but not in dialysis, the probability of postponing the dialysis in 3y time was not modified. CONCLUSIONS: This model did show a relevant improvement with ERT for those that were treated at initial stages of renal disease (the subgroup with proteinuria only). For this group the probability of remaining in a less severe stage of the disease in a 3y time, postponing renal impairment was significant for those using ERT. This was only a preliminar model, considering that the maintenance of less severe states of renal disease postpone the need for dialysis in this sub-group, aspects such as quality of life will be added in order to do a more complete evaluation. This model was build considering clinical outcomes, something new on rare diseases. The discussion on how to evaluate and on the value of ‘classical modeling’ in rare diseases are still an on going issue with a lot to be elucidated.