We have investigated the phase behavior of a system of hard cylinders in the limit L/D ®0. Simulations in this limit are possible because the model can be mapped onto a system of particles with finite volume but variable shape. Our simulations indicate that infinitely thin disks exhibit a columnar phase, and that the nematic-columnar transition is first order. Combining this result with previous simulations of cut spheres, this transition appears to be first order for all aspect ratios 0<L/D£ 0.1.