A dishwasher and a method for controlling the same, performing a main washing cycle depending upon an amount of food residue on used dishes or a pollution level thereof, are disclosed. The method includes supplying washing water to the dishwasher, detecting for an N number of times a pollution level of the supplied washing water during a preliminary washing cycle, and comparing the detected pollution levels with a reference pollution level, and carrying out a main washing cycle depending upon a comparison result between the detected pollution levels and the reference pollution level.