Fragility functions are determined for low- and mid-rise ordinary concrete buildings, which constitute the most vulnerable construction type in Turkey as well as several other countries prone to earthquakes. A hybrid approach is employed where building capacities are obtained from field data and their dynamic responses are calculated by response history analyses. Field data consists of 32 sample buildings representing the general characteristics of two-to five-story substandard reinforced concrete buildings in Turkey. Lateral stiffness, strength, and deformation capacities of the sample buildings are determined by pushover analyses conducted in two principal directions. Uncertainties in lateral stiffness, strength, and damage limit states are expressed by using statistical distributions. The inelastic dynamic structural characteristics of the buildings investigated are represented by a family of equivalent single-degree-of-freedom systems and their seismic deformation demands are calculated under 82 ground-motion records. Peak ground velocity (PGV) is selected as the measure of seismic intensity since maximum inelastic displacements are better correlated with, PGV than peak ground acceleration (PGA). Fragility functions are derived separately for different number of stories, which is a prominent parameter influencing the vulnerability of existing substandard concrete buildings.