  Wilson and Bappu (1957) have shown the existence of a remarkable correlation
between the width of the emission in the core of the K line of Ca II and the
absolute visual magnitude of late--type stars. Here we present a new
calibration of the Wilson--Bappu effect based on a sample of 119 nearby stars.
We use, for the first time, width measurements based on high resolution and
high signal to noise ratio CCD spectra and absolute visual magnitudes from the
Hipparcos database. Our primary goal is to investigate the possibility of using
the Wilson--Bappu effect to determine accurate distances to single stars and
groups. The result of our calibration fitting of the Wilson--Bappu relationship
is M_V=33.2-18.0*log W_0, and the determination seems free of systematic
effects. The root mean square error of the fitting is 0.6 magnitudes. This
error is mostly accounted for by measurement errors and intrinsic variability
of W_0, but in addition a possible dependence on the metallicity is found,
which becomes clearly noticeable for metallicities below [Fe/H]=-0.4. This
detection is possible because in our sample [Fe/H] ranges from -1.5 to 0.4. The
Wilson--Bappu effect can be used confidently for all metallicities not lower
than -0.4, including the LMC. While it does not provide accurate distances to
single stars, it is a useful tool to determine accurate distances to clusters
and aggregates, where a sufficient number of stars can be observed. We apply
the Wilson--Bappu effect to published data of the open cluster M67; the
retrieved distance modulus is of 9.65 magnitude, in very good agreement with
the best distance estimations for this cluster, based on main sequence fitting.
