Identifying gene-specific alterations in cancer genomes has revealed molecules that are causal effectors of carcinogenesis and specific targets for cancer molecular diagnosis and molecular-based cancer therapies. Whole-genome analyses of many cancer genomes at the resolution of single genes is thus a desirable yet incompletely realized goal that could expedite progress in cancer diagnosis and treatment. Although methods for routine whole-genome sequencing or high-resolution epigenetic measurements are currently under development, high-resolution measurements of gene copy number, or 'gene dosage', are now underway in several laboratories. Digital karyotyping, array comparative genomic hybridization, and single nucleotide polymorphism arrays are techniques that have the potential to detect gene amplification, homozygous deletion and loss of heterozygosity at or below the average length of single genes. Recently, digital karyotyping of a small number (<20) of colon and brain cancer genomes has revealed tumor cases with significant genetic dosage alterations affecting few and, in some cases, only one complete gene. These experiments suggest that gene-specific gene dosage alterations may be sufficiently frequent to enable the identification of promising tumor gene candidates in small-scale experiments. The purpose of this review is to describe our understanding of cancer as a genetic disease, review the basic principles, methodologies and interpretational issues of traditional and high-resolution whole-genome screens, and describe the potential of our first detailed look at whole cancer genomes for progress in the understanding and treatment of cancer.