A search for new massive resonances in the dielectron and dimuon decay channels has been performed combining 2015 data collected by the CMS experiment in proton - proton collisions at a center of mass energy of 13 TeV (corresponding to an integrated luminosity of 2.9 fb−1) with a previous analysed set of data obtained at 8 TeV (corresponding to 20 fb−1). The analysis has been updated with data collected during Run II in 2016 at 13 TeV (13 fb−1). In the absence of a significant deviation from the standard model predictions, 95% confidence level limits are set on the ratio of the production cross section times branching fraction for possible new high-mass resonances to that for the Z boson. Limits are set on the masses of hypothetical particles that could appear in new-physics scenarios.