In support of the initiatives for implementing the new Mechanistic- Empirical Pavement Design Guide (MEPDG) in Iowa, a preliminary sensitivity study was undertaken to assess the comparative effect of design input parameters pertaining to material properties, traffic and climate on performance of two existing flexible pavements in Iowa with relatively thick Asphalt Concrete (AC) layers. A total of 20 individual inputs were evaluated by studying the effect of each input on five different performance measures (longitudinal cracking, alligator cracking, transverse cracking, rutting, and roughness) for each pavement structure resulting in about 200 simulations using the MEPDG software. The results showed that the predicted longitudinal cracking and total rutting were influenced by most input parameters. Alligator cracking, transverse cracking, and roughness remained insensitive to most input parameters. Future research will focus on comparing the predicted measures against the recorded pavement distresses in the Iowa DOTs Pavement Management Information System (PMIS) database.