Recent research has addressed the problem of providing automated assistance to software developers in classifying reported instances of software failures so that failures with the same cause are grouped together. In this paper, two new tree-based techniques are presented for refining an initial classification of failures. One of these techniques is based on the use of dendrograms, which are rooted trees used to represent the results of hierarchical cluster analysis. The second technique employs a classification tree constructed to recognize failed executions. With both techniques, the tree representation is used to guide the refinement process. We also report the results of experimentally evaluating these techniques on several subject programs.