The paper deals with locally finite groups G having an involution φ such that C G (φ) is of finite rank. The following theorem gives a very detailed description of such groups.Let G be a locally finite group having an involution φ such that C G (φ) is of finite rank. Then G/ [G, φ] has finite rank. Furthermore, [G, φ] contains a characteristic subgroup B such that the following hold.(1) B is a product of finitely many subgroups normal in [G, φ] isomorphic to either PSL(2, K) or SL(2, K) for some infinite locally finite fields K of odd characteristic.(2) [G, φ] /B has finite rank.