Enzyme therapy has proven safe and effective in preventing and reversing many manifestations in patients with Gaucher disease. On the basis of this success, enzyme therapy is now becoming a reality for Fabry disease, alpha-galactosidase A deficiency. Two products, agalsidase alpha and beta, have been tested in pivotal trials. The substantial differences between the study structures and outcome measures have made direct comparisons difficult. Here, the strengths and weaknesses of these trials are compared: achievement of stated endpoints, safety and, potential efficacy. In addition, the need for additional long-term data is emphasized because this is not attainable in short-term trials for a chronic disease.