The Hartree-Fock treatment of cyclotron resonance (CR) [see A. H. MacDonald, J. Phys. C 18, 1003 (1985)] is generalized by including the correlation contribution to the screening by the electrons. It is found that the CR effective mass m* is an oscillatory function of the magnetic field, which is due to the filling-factor-dependent dynamic screening of the electron-impurity scattering. It is demonstrated that as the Landau levels are depleted with increasing magnetic field, m* has a maximum at half-filling. Similar behavior has also been found in CR absorption of the two-dimensional electron gas in a high-mobility heterostructure.