[en] A study of events with isolated leptons and missing transverse momentum in ep collisions is presented. Within the Standard Model (SM) such topologies are expected mainly from production of real W bosons with subsequent leptonic decay. This thesis continues the analysis of such events done in the HERA-1 period where an excess over the SM prediction was observed for events with high hadronic transverse momentum P^X_T>25 GeV. New data of the HERA-2 period are added. The analysed data sample recorded in e⁺p collisions corresponds to an integrated luminosity of 220 pb^-1 which is a factor of two more with respect to the HERA-1 analysis. The e^-p data correspond to 186 pb^-1 which is a factor of 13 more with respect to HERA-1. All three lepton generations (electrons muons and tau leptons) are analysed. In the electron and muon channels a total of 53 events are observed in 406 pb^-1. This compares well to the SM expectation of 53.7±6.5 events, dominated by W production. However a difference in the event rate is observed for different electron beam charges. In e⁺p data the excess of events with P^X_T>25 GeV is sustained, while the e^-p data agree with the SM. In the tau channel 18 events are observed in all HERA data, with 20±3 expected from the SM. The events are dominated by irreducible background from charged currents. The contribution from W production amounts to about 22%. One event with P^X_T>25 GeV is observed, where 1.4±0.3 are expected from the SM. (orig.)