Total and single-differential cross sections for electron-impact ionizaton are calculated in a relativistic formulation for ions in the hydrogen isoelectronic sequence: Hi i, He ii, C vi, Ne x, Fe xxvi, and Ag xlvii. Transition amplitudes are evaluated in the two-potential distorted-wave approximation. Sets of different asymptotic charges are used to study the mutual screening of the primary and secondary electrons. Relativistic effects are investigated by taking the nonrelativistic limit and are found to increase the cross sections. Thomson’s scaling law along the isoelectronic sequence is also studied.