Detection of copy number variation in the human genome is important for identifying naturally occurring copy number polymorphisms as well as alterations that underlie various human diseases, including cancer. Digital karyotyping uses short sequence tags derived from specific genomic loci to provide a quantitative and high-resolution view of copy number changes on a genome-wide scale. Genomic tags are obtained using a combination of enzymatic digests and isolation of short DNA sequences. Individual tags are linked into ditags, concatenated, cloned and sequenced. Tags are matched to reference genome sequences and digital enumeration of groups of neighboring tags provides quantitative copy number information along each chromosome. Digital karyotyping libraries can be generated in about a week, and library sequencing and data analysis require several additional weeks.