In vitro cultivated cells of 28 colorectal cancers were analyzed for chromosomal abnormalities that might signal amplification of DNA, either as double minutes (DMs) or homogeneously staining chromosomal regions (HSRs). Cells derived from 18 tumors showed DMs in 10 to 100% of all metaphases examined. Surveys that employed a panel of available oncogene probes failed to detect amplification of a known cellular oncogene with the exception of three cases where the ERBB2 gene was amplified. In one of these three cases neither DMs nor HSRs were detectable. Our studies show that from 28 lines established in culture, 19 (68%) show amplification of DNA, and indicate that DNA amplification is a frequent genetic alteration in colorectal cancers in addition to other genetic changes. Amplification is correlated with high Dukes stage, but not with histological grade. The identity of the amplified DNA remains to be established for most cases.