Journal of Research in Medical Sciences1735-1995822003062829892989ENDepartment of Biostatistics and Epidemiology, School of Health, Isfahan University of Medical Scienc, Isfahan,Iran2003062820090208The problem of carry-over effect in two period -two sequence cross-over studies has already brought about a lot of discussion and has induced to bring up different views and recommendations. The present study deals with relationship between carry-over effects and sample size in 2 x 2 cross-over design. It has been shown that when such effects exist, number of samples will differ from the case when there is no carry-over effect. The difference in sampel size has been estimated and has been shown that depends upon the differences in size and direction of carry-over effect and direct treatment effects. The influence of carry-over effects on power of the test also has been considered and its variation has been shown. The results suggest that while determining sampel size for a 2x2 cross-over study, it is matter of wise to take into account any knowledge about carry-over effect difference which is in hand from a previous or pilot study or even from a rational guess, especially if carry-over effects difference and direct treatment effects difference are in the same direction. Theoretical outcomes are used for a practical example and related results have been presented.http://jrms.mui.ac.ir/index.php/jrms/article/view/2989