In this paper, we describe the use of a modern learning classifier system to a data mining task. In particular, in collaboration with a medical specialist, we apply XCS to a primary breast cancer data set. Our results indicate more effective knowledge discovery than with C4.5. Categories and Subject Descriptors I.2.8 [Artificial Intelligence]: Problem Solving, Control Methods and Search – backtracking, control theory, dynamic programming, graph and tree search strategies, heuristic methods, plan execution formation and execution, scheduling. General Terms Algorithms, Measurement, Performance, Experimentation. Keywords Classification, Genetic Algorithm, Learning Classifier System, Medical Informatics.