Hipparcos parallaxes were used to derive absolute visual magnitudes of G, K, and M stars with Ca II emission line widths previously measured by O.C. Wilson. A linear relationship similar to the one derived originally by Wilson & Bappu and improved by Lutz & Kelker was found from M_v_=+7 to -2. For stars brighter than M_v_=-2 a substantial number of stars show Ca II emission lines that are broader than expected from the linear fit. Most of those stars are bright giants and supergiants of type G. Introduction: In 1957 O. C. Wilson and M. K. V. Bappu reported on the remarkable correlation between the measured width of the emission feature at the center of the Ca II K line and the absolute visual magnitude of the star. The correlation is independent of spectral type and applicable to stars of type G, K, and M. Their original calibration was based on the Sun and the four K giants in the Hyades. Except for the main-sequence stars there were not many additional calibrating stars with sufficiently accurate parallaxes to evaluate the dispersion about the mean relation or to extend the calibration to stars brighter than about M_v_ = 0. The paucity of accurate parallaxes for luminous red giants has been largely overcome by the Hipparcos program (Kovalevsky 1998ARA&A..36...99K) in which parallaxes with uncertainties of near 1 mas were obtained for numerous stars. Hence, we now have 10 sigma parallaxes out to 100 pc and 5 sigma data to 200 pc, providing M_v_-values with uncertainties of 0.21 and 0.42 mag, respectively. Therefore, the large database provided by Wilson (1976ApJ...205..823W) can be combined with Hipparcos parallaxes to recalibrate the M_v_ - log W_0_ relation, where W_0_ is the measured width corrected for the instrumental width. In addition, stars that appear to deviate from the Wilson-Bappu relation may be investigated in order to understand the anomalies of their chromospheres. We have limited ourselves to stars with parallaxes that are at least 5 times their probable error and whose log W_0_-values are listed by Wilson (1967PASP...79...46W, 1976ApJ...205..823W). Other databases are available, e.g., Warner (1967MNRAS.137..119W) and Zarro & Rogers (1983ApJS...53..815Z), but the Wilson data are so much larger than the others that we decided to base our discussion on these data alone. For each star we have extracted the parallax, its probable error, V magnitude, spectral type, B-V color, and V-I color from the Hipparcos Catalogue (ESA 1997, Cat. ). These quantities are listed in Table 1 along with Wilson's intensity index (1-5), log W_0_, and the M_v_-value derived from the apparent magnitude and parallax. Table 1 is divided into three sections: Table1a.dat contains stars with Ca II emission index 2 from Wilson (1976ApJ...205..823W); Table1b.dat contains stars of index 1 from the same source; and Table1c.dat contains stars in Wilson (1967PASP...79...46W). The complete data set has been used to plot fig1.gif, where we show M_v_ plotted against log W_0_, the same coordinates used by Wilson & Bappu (1957ApJ...126...46W). Because of the crowding we do not show the probable errors of each point in this plot.
