Fission rate monitor measurements and startup testing data recorded during operation of the Advanced Test Reactor (ATR) have been used to validate a three-dimensional PDQ full-core model developed for the physics analysis to support an updated final safety analysis report. The three-dimensional analysis utilizes the flux synthesis technique as well as the explicit method in solving for the spatial flux distribution in the core. Measured data used for comparison are specific powers from a string offission rate monitors, located in water channels of individual fuel elements, as well as the lobe and fuel element powers. Good agreement was observed in the specific power comparison. For the overall pointwise data, the mean errors were within 1.6% with a standard deviation of ±9%. An excellent agreement was observed for the fuel element power except for a few fuel positions in the corner lobes. Measured ATR startup testing data are also compared with the PDQ calculated values. The PDQ calculated parameters were conservative with respect to measured data. The validation study provided valuable data for assessment of the three-dimensional analytical model and techniques to be employed in the ATR physics analysis. The study also indicated that the PDQ three-dimensional flux synthesis solution technique is an economical and reasonably accurate method for determining global and local three-dimensional power distributions in the core.