A probabilistic approach to the lifetime seismic assessment of deteriorating concrete structures is presented. The seismic performance is evaluated by incremental dynamic analysis over the structural lifetime. The uncertainties involved in the problem are taken into account by Monte Carlo simulation and fragility analyses are performed with respect to different limit states, from immediate occupancy up to collapse. The time-variant mean annual frequencies of exceedance are computed based on the seismic hazard of the site. The application to the lifetime seismic assessment of a three-story reinforced concrete building exposed to corrosion shows the effectiveness of the proposed methodology.