  For the past twenty years, it has been accepted that the vertical scaleheight
of the stellar disk in spiral galaxies is constant with radius. However, there
is no clear physical explanation for this in the literature. Here we calculate
the vertical stellar scaleheight for a self-gravitating stellar disk including
the additional gravitational force of the HI and H_2 gas and the dark matter
halo. We apply our model to two edge-on galaxies, NGC 891 and NGC 4565, and
find that the resulting scaleheight shows a linear increase of nearly a factor
of two within the optical disk for both these galaxies. Interestingly, we show
that the observed data when looked at closely, do not imply a constant
scaleheight but actually support this moderate flaring in scaleheight.
