  We examine all possible stationary, optically thick, geometrically thin
accretion disc models relevant for active galactic nuclei (AGN) and identify
the physical regimes in which they are stable against the thermal-viscous
hydrogen ionization instability. Self-gravity and irradiation effects are
included. We find that most if not all AGN discs are unstable. Observed AGN
therefore represent the outburst state, although some or all quasars could
constitute a steady population having markedly higher fuelling rates than other
AGN. It has important implications for the AGN mass supply and for the presence
of supermassive black holes in nearby spirals.
