Finally, we regret not indicating that the studies of 5-HT2A receptor density referenced in the article were cited as examples, as we were unable to cite all studies because of the space limitations of a Brief Report. We have read all of the studies cited by Dr. Mendelson (Mendelson, 2000), and none of them mentioned smoking. More important, Dr. Mendelson ignores our study (4, cited in the article) that found a marked, dose-dependent elevation of 5-HT2A receptor density among smokers independent of depressive symptoms. When the results of both of our studies are taken together with the fact that one-half of clinically depressed patients are current smokers (5), it is entirely possible (and, in our view, likely) that all previous studies have been confounded by smoking. Indeed, this factor may explain, at least in part, the inconsistencies in these studies described by Dr. Mendelson as puzzling (Mendelson, 2000). It is unfortunate that the investigators in the area have not heeded the advice of Glassman (6), who stated that "at the simplest level, psychiatric research needs to control for smoking."