Long-term light curve of M33 X-8. Each point corresponds to a single observation, and fluxes are not corrected for absorption. The luminosity on the right axis has been evaluated assuming a distance of 820 kpc. We have associated an arbitrary 5% statistical error to the BeppoSAX, Chandra, and Suzaku points. The horizontal line represents the Eddington luminosity for a 10 M_⊙ black hole.