In Reply.
—As suggested by Drs Murray and Steck, our recipients' high prestudy exacerbation rate might have been expected to regress to the mean rate of the total population during the study. However, the dramatic decrease in their rate—from greater than that of the controls before the study to less than that during it—would not be expected to occur naturally. The idea that the decrease in recipients' exacerbations reflected a "burn out" during the normal course of the disease is not supported by observations on the annual relapse rates in 393 cases of MS.1 These observations show that at the stage at which our recipients entered the study (after a mean duration of illness of 8.1 years), a maximum decrease in relapse rate of 16% during the study might have been attributed to the disease's natural course. We observed a decrease of 88%. We presented the Mann-Whitney P values