We investigate the influence of blending on the Cepheid distance scale .
Blending leads to systematically low distances to galaxies observed with HST , and therefore to systematically high estimates of the Hubble constant H _ { 0 } .
We select a sample of 43 long-period , large-amplitude Cepheids in the Large Magellanic Cloud , from the catalog recently released by the Optical Gravitational Lensing Experiment .
We then model the effects of blending , as observed by HST at large distances , by adding the V,I -bands contributions from nearby bright stars .
We find that the derived distance modulus would be too short , compared to the true value , by \sim 0.07 mag at distance of 12.5 Mpc and by \sim 0.35 mag at distance of 25 Mpc .
This has direct and important implications for the Cepheid distances to galaxies observed by the HST Key Project on the Extragalactic Distance Scale and other teams : half of the KP sample is likely to exhibit a blending bias greater than 0.1 mag .