To the Editor.
—Dr Esdaile's recent article entitled "A Reappraisal of the United Kingdom Epidemic of Fatal Asthma,"1 although interesting, leaves out one important fact and begs the real issue faced at the time the epidemic was being studied. Much of what is indicated in their article about the difficulties in using ecologic relationships to identify causation was, in fact, summarized in our original publication.2 It was for this reason that the investigative study3 was carried out. In that study, a point not commented on by Esdaile and coworkers was the fact that of the 113 cases in which an autopsy was performed, 110 of the cases had classic pathologic evidence that the appropriate diagnosis was asthma. These findings would seem to refute their speculative argument that "diagnostic exchange" might have accounted for the epidemic.We would agree that the best way to study therapeutic agents is