We report the first evidence of increased levels of the retinoblastoma (Rb) message in a majority of col-orectal cancers when compared with normal mucosa. Southern blot analysis showed an increase in Rb gene copy number in at least 28% of colorectal carcinomas relative to normal mucosa. These results plus previous reports of nonrandom chromosome 13 gains in approximately 50% of colorectal cancers suggest that an increase in Rb gene copy number occurs frequently in these tumors. Possible mechanisms pertaining to overexpression of the Rb gene are discussed in relation to its role as a recessive cancer gene. [J Natl Cancer Inst 82:310–314, 1990]