Achondroplasia is an autosomal dominant disorder that involves defective endochondral bone formation. Type II collagen is the predominant collagen of cartilage. We found a HindIII polymorphic site in the normal Caucasian population by using the type II procollagen gene probe pgHCol(II)A. The presence of this site yields a 7.0-kilobase (kb) band; its absence yields a 14.0-kb band. We found a significant deviation in genotype distribution and allele frequencies in a population of unrelated individuals with sporadic achondroplasia, compared with the normal control population. The HindIII genotype frequencies in 32 individuals with achondroplasia are 0.41 for the 7/7 genotype (controls, 0.08), 0.34 for the 7/14 genotype (controls, 0.54), and 0.25 for the 14/14 genotype (controls, 0.37). The apparent equilibrium excess of the "7" allele in individuals with achondroplasia may reflect either a predisposition for the mutation that causes achondroplasia or it could be the result of the achondroplasia-causing mutation. In either case, these findings suggest an association of the type II procollagen gene with achondroplasia.