The experimental demonstration of a free-space optical transmission system with orbital angular momentum (OAM) division multiplexing is reported. Two beams have been multiplexed with different values of the azimuthal index l, and hence carrying different OAM, at a wavelength of 1550 nm. The first beam (l = 0) is nearly Gaussian and it features an OAM equal to zero. The second beam (l = 1) is an optical vortex produced by means of a suitable computer-generated hologram, which shows an intensity pattern with a doughnut shape and possesses an OAM of -h per photon. A binary intensity modulation at 1.25 Gbit/s has been applied to each OAM channel and an interferometer with Dove prisms has been exploited for OAM demultiplexing.