Chromosome analysis of blood cells from a 42-year-old white male with mental retardation, colon carcinoma, horseshoe kidney, absence of left lobe of the liver, agenesis of the gallbladder, and possible Gardner syndrome revealed a constitutional marker chromosome due to del(5)(q13q15) or del(5)(q15q22). A polymorphic chromosome #22 with enlarged satellites was inherited from the father, who is phenotypically normal, and was probably unrelated to the congenital malformations. This is the first report of a Gardner syndrome patient with an interstitial deletion of 5q.