A large scale seismic risk prioritization study was conducted for medium-rise, gravity-designed reinforced concrete residential buildings in Istanbul. The implemented procedure was based on evaluating selected building parameters that could be easily observed or measured during a systematic sidewalk survey. The survey was actually conducted on approximately 100,000 buildings in the six subprovinces of Istanbul over the period 20042009 by a large team of trained surveyors. The collected data were processed to calculate a safety score for each building, which is in turn used for ranking the surveyed buildings with respect to their expected seismic performance under the calculated ground motion intensities produced for a scenario earthquake of M7.5 along the Marmara Sea Fault. The basic objective of this laborious study was to identify the buildings that would be likely to sustain severe damage or suffer collapse during the expected Istanbul earthquake. Accordingly, a rational risk reduction planning can be conducted for minimizing the life and property losses. This paper presents the methodology, results of field surveys and risks evaluated in 100,000 buildings. As a result, 52,000 buildings have been classified as high risk, that is, expected to sustain severe damage or collapse during the scenario event. Copyright (C) 2012 John Wiley & Sons, Ltd.