We show that the equivalent widths of the well-known interstellar CaII H and K lines can be used to determine the distances to OB stars in our Galaxy. The equivalent widths, measured in the spectra of 147 early-type stars, are strongly related to the Hipparcos parallaxes of those objects. The lines fitted to the parallax-equivalent width data are given by the formulae {pi}=1/[2.78EW(K)+95] and {pi}=1/[4.58EW(H)+102], where {pi} is in arcseconds and EW is in milliangstroms.