Broadly speaking, the present invention fills the need of selecting correlated advertisements for displaying to users by utilizing the advertisement-viewing data collected on users of a web site. The advertisement-viewing (ad-viewing) data of users of a web site can be correlated to extract similarities and patterns of ads being viewed by users of the web site. The correlated ad-viewing data of all users of the web site, along with URLs (Uniform Resource Locators) of ads viewed by a particular user, can be used to select advertisements that are likely to be interests to the particular user. Ad-viewing data correlation allows the web site to select ads to display to users in real-time based on users' latest ad-viewing data. Since selection of ads to display is based on mathematical calculation using a correlation table of ad-viewing that has been generated ahead of time, the method can be scaled to meet demands of a large amount of users, such as millions. In addition to ad-viewing correlation table and users' latest ad-viewing data, advertisement pool of advertisements paid by advertisers is also taken into consideration during the ad-selection process.