We analyze the influence of atmospheric turbulence on the propagation of an optical vortex beam having the form $V(r,\theta )=A_0{e}^{im\theta }$ . The probability that a detected photon after propagating through the atmosphere has the same value of the orbital angular momentum as the launched photon is found to be given by $⟨s_0⟩={[1+{(1.845D∕r_0)}^2]}^{-1∕2}$ , where D is the aperture diameter and $r_0$ is the Fried coherence diameter. These vortex beams behave very similarly to Laguerre–Gauss beams under the influence of atmospheric turbulence. These results have important implications for atmospheric laser communication systems that employ quantum encryption.