  Giant low surface brightness galaxies (GLSBs), such as Malin 1, have
unusually large and flat discs. Their formation is a puzzle for cosmological
simulations in the cold dark matter scenario. We suggest that GLSBs might be
the final product of the dynamical evolution of collisional ring galaxies. In
fact, our simulations show that, approximately 0.5-1.5 Gyr after the collision
which lead to the formation of a ring galaxy, the ring keeps expanding and
fades, while the disc becomes very large (~100 kpc) and flat. At this stage,
our simulated galaxies match many properties of GLSBs (surface brightness
profile, morphology, HI spectrum and rotation curve).
