Let H be either a complex inner product space of dimension at least two, or a real inner product space of dimension at least three. Let us fix an α ∈ 0, π 2 . The purpose of this paper is to characterize all bijective transformations on the projective space P (H) obtained from H which preserves the angle α between lines in both directions. (We emphasize that we do not assume anything about other angles). For real inner product spaces and when H = C 2 we do this for every α, and when H is a complex inner product space of dimension at least three we describe the structure of these transformations for α ≤ π 4 . As an application, we give an Uhlhorn-type generalization of a famous theorem of Wigner which is considered to be a cornerstone of the mathematical foundations of quantum mechanics. Namely, we show that under the above assumptions, every bijective map on the set of pure states of a quantum mechanical system that preserves the transition probability cos 2 α in both directions is a Wigner symmetry (i.e. it automatically preserves all transition probability), except for the case when H = C 2 and α = π 4 where an additional possibility occurs. We note that the classical theorem of Uhlhorn is the solution for the α = π 2 case. Usually in the literature, results which are connected to Wigner's theorem are discussed under the assumption of completeness of H, however, here we shall remove this unnecessary hypothesis. Our main tools are a characterization of bijective maps on unit spheres of real inner product spaces which preserve an angle in both directions, and an extension of Uhlhorn's theorem for non-complete inner product spaces.2010 Mathematics Subject Classification. Primary: 47B49, 47N50, 81P05, 51A05.