Several earthquake-prone countries including Turkey do not consider vertical mass discontinuity as a type of structural irregularity in their seismic codes. However, other earthquake prone Countries clearly explain the mass irregularity and have limitations on the use of approximate methods to determine the earthquake forces developed in an irregular structure. The applicability of equivalent lateral force procedure of the Turkish earthquake code for mass irregular buildings is examined in this study. This is achieved by comparing the approximate results to those obtained by linear and non-linear time history analyses. 75 real and 100 simulated earthquake records are used in time history analyses. Two-dimensional 5, 10, and 20-story Structures idealized as shear and frame buildings are studied. Mass irregularity is resulted from varying the mass of one floor and keeping the other story masses constant. Effect of altering the mass of different floors is also investigated. The comparison of the analysis results shows that the approximate method always overestimates the linear behaviour regardless of structure height, building rigidity and degree of mass irregularity. The method, however, underestimates the non-linear response of story columns. In light of the results obtained in this study, the inclusion of mass irregularity to the code appears appropriate.