Liou et al then go on to present data that seemingly respond to this need by interrogating the Cystic Fibrosis Foundation Patient Registry and conclude that our model was useless in predicting actual changes in FEV1 percent predicted (Fig 1 in their letter). We are reluctant to engage in a dispute about the meaning of their results because in a letter to the editor too little information is available about the methods employed. For instance, legitimate concerns about data quality are raised by Liou et al themselves in their letter. Furthermore, at the very least, it would be important to exactly replicate the methods used in our study before concluding that there were or were not meaningful differences. We have real concerns that Liou et al did not do this. For instance, in our study, 54% of the patients were in the “slow group” showing pulmonary function decline (ie, the rate of decline for each patient was less than the group mean + 2 SEs). The coefficient of determination for the linear regressions in this group averaged only 0.44 ± 0.30. Clearly, linear regressions in such patients cannot predict future events (in this case, for instance, the FEV1 percent predicted for the next year) with great confidence. Of course, by definition, these patients show little change in their pulmonary function, and therefore, with all other things being equal, they would not be likely candidates for lung transplantation referral. We never implied that such patients do not require close clinical observation and follow-up. If circumstances change, the clinician must adapt accordingly.