Because of the clinical and histopathologic similarities between otosclerosis and type I osteogenesis imperfecta, we examined COL1A1 messenger RNA (mRNA) expression in cultured fibroblasts from patients with clinical otosclerosis to determine whether abnormalities of expression of COL1A1 were present, as has been reported in type I osteogenesis imperfecta. Type I osteogenesis imperfecta has been found to result from mutations in the COL1A1 gene that result in null expression of the mutant allele. Patients with clinical otosclerosis were genotyped for the presence of an expressed 4 base-pair insertion polymorphism in the 3' region of the COL1A1 gene. Skin biopsies were performed, and cultured fibroblast cell lines were established from patients who were heterozygous for the polymorphism. Allelic expression was examined by reverse transcription-polymerase chain reaction and silver-stained polyacrylamide gel electrophoresis. Two of 9 patients with clinical otosclerosis demonstrated null or reduced expression of one COL1A1 allele. The differential expression of the two COL1A1 alleles in all subjects was also examined by a semiquantitative method using an ABI Prism 7700 Sequence Detection System (Taqman). We did this examination to determine whether milder abnormalities in COL1A1 expression might account for the development of otosclerosis in the 7 clinical cases that did not reveal evidence of null expression by the gel technique. Of the same 2 cases of otosclerosis that demonstrated evidence of null expression by gel electrophoresis, both were found to have significant differences in COL1A1 mRNA expression by the Taqman analysis. The remaining 7 cases revealed equal expression of the two COL1A1 alleles similar to that seen in controls. These results suggest that mutations in COL1A1 that are similar to those that occur in type I osteogenesis imperfecta may account for a small percentage of cases of otosclerosis, and that the majority of cases of clinical otosclerosis are related to other genetic abnormalities that have yet to be identified.