We describe Syskill and Webert, a software agent that learns to rate pages on the World Wide Web (WWW), deciding what pages might interest a user. The user rates explored pages on a three point scale, and Syskill and Webert learns a user profile by analyzing the information on each page. We focus on an extension to Syskill and Webert that lets a user provide the system with an initial profile of his interests in order to increase the classification accuracy without seeing many rated pages. We represent this user profile in a probabilistic way, which allows us to revise the profile as more training data is becoming available using "conjugate priors", a common technique from Bayesian statistics for probability revision. Unseen pages are classified using a simple Bayesian classifier that uses the revised probabilities. We compare our approach to learning algorithms that do not make use of such background knowledge, and find that a user defined profile can significantly increase the classification accuracy.