Microarray technology allows to measureing the expression levels of thousands of genes in an experiment. This technology required requires computational solutions capable of dealing with great amounts of data and as well as techniques to explore the data and extract knowledge which allow patients classification. This paper presents a systems based on Case-based reasoning (CBR) for automatic classification of leukemia patients from microarray data. The system incorporates novel algorithms for data mining that allow to filter and classify as well as extraction of knowledge. The system has been tested and the results obtained are presented in this paper.