Array-based comparative genomic hybridization (aCGH) is a powerful assay to identify copy number abnormalities underlying the pathogenesis of cancer. aCGH has become the gold standard for whole genome copy number analysis in medium and large cohorts in clinical and research laboratories. Identifying the best workflow is critical to achieving the optimal performance for this assay. Here we describe the aCGH protocol used by our group in the study of B-chronic lymphocytic leukemia (CLL). We also describe some initial applications of aCGH in association with clinical outcome for CLL.