In M dwarfs the CaI 4227 line is saturated, therefore, the line core forms at a relatively small column mass density. As a result, the line is a potential diagnostic of chromospheric structure in these stars. We have caluclated line profiles out of LTE for a grid of early M dwarf atmospheric models that spans the range from low to high chromospheric pressure and covers a range of observed activity levels. We have found that this line is sensitive to the detailed structure of the chromospehre in the same way as well studied strong lines such as H alpha and CaII HK: as the chromospheric pressure increases, the line changes from absorption to strong emission with a central double reversal. Therefore, observations of the CaI 4227 line in M dwarfs would provide a useful additional constraint on the structure of the outer atmosphere. We also calculate consistently the line blanketing in the upper atmosphere of models with a chromosphere and transition region and investigate the effect on its inclusion on CaI 4227. Finally, we demonstrate that the effects of Partial Frequency Redistribution must be taken into account if the CaI 4227 is to be used as an accurate diagnostic of the T_min region in dMe stars.