In patients with osteogenesis imperfecta (OI) type I, a decrease in synthesis of type I collagen is usually observed as a result of a COL1A1 null allele. Testing for COL1A1 null alleles can be done using polymorphic markers in the coding region of the COL1A1 gene. Until now, only one marker for polymorphism in the 3' untranslated region (3' UTR) of the COL1A1 gene has been available. We have identified a 4 bp insertion in the 3' UTR of the COL1A1 gene localized downstream of the MnlI RFLP and used both markers in combination for the analysis of patients with OI type I. In a total of 50 patients, 28 showed heterozygosity for one of the two markers; 14 of them were shown to have a COL1A1 null allele.