Overall, these data strongly suggest that agalsidase-beta ERT slows the decline of glomerular function in patients with Fabry disease. Banikazemi and colleagues' study is the first controlled trial to show an effect of ERT on a major complication of Fabry disease, but the study was underpowered, and we are left with some uncertainty about whether the differences were due to chance or to agalsidase-beta ERT. If the study was performed in a larger study sample, the effect would probably have been more conclusive by statistical criteria, but we can't be sure. Nevertheless, we can estimate that the effect size of agalsidase beta was rather large when compared with the renal-sparing effect of angiotensin-converting enzyme (ACE) inhibitors and angiotensin-receptor blockers (ARBs). Using similar study structures, other studies have found that 583 patients, who were followed for 3 years, were needed to demonstrate a 50% risk reduction with benazepril (11) and that 1513 patients, who were followed for an average of 3.4 years, were needed to show a 16% mean risk reduction with losartan (12). The authors' choice of an outcome measure may have made detection of an effect of ERT more difficult. Categorical efficacy measures, such as clinical events, are clinically meaningful but are known to be less sensitive than continuous measures, such as the mean rate of change in GFR (13).