Array-based comparative genomic hybridization (CGH) uses multiple genomic clones arrayed on a slide to detect relative copy number of tumor DNA sequences. Application of array CGH to tumor specimens makes genetic diagnosis of cancers possible and may help to differentiate relevant subsets of tumors, biologically and clinically, which would allow better prognostic and therapeutic decision making. In this study, we have used array-based CGH to detect DNA copy number alterations in distinct types of renal cell carcinomas for diagnostic purposes. We were able to correctly diagnose 33 of 34 malignant tumors by automated computational means and to group together eight benign neoplasms and normal kidney samples. These results indicate that array-based CGH is capable of diagnosing the vast majority of renal cell carcinomas based on their genetic profiles.