Osteogenesis imperfecta (OI) is a heritable disorder with bone fragility that is often associated with short stature, tooth abnormalities (dentinogenesis imperfecta), and blue sclera. The most common mutations associated with OI result from the substitution for glycine by another amino acid in the triple helical domain of either the a1 or the a2 chain of collagen type I. In this study, we compared the results of genotype analysis and clinical examination in 161 OI patients (median age: 13 years) who had glycine mutations in the triple helical domain of a1(I) (n¼67) or a2(I) (n¼94). Serine substitutions were the most frequently encountered type of mutation in both chains. Compared with patients with serine substitutions in a2(I) (n¼40), patients with serine substitutions in a1(I) (n¼42) on average were shorter (median height z-score À6.0 vs À3.4; P¼0.005), indicating that a1(I) mutations cause a more severe phenotype. Height correlated with the location of the mutation in the a2(I) chain but not in the a1(I) chain. Patients with mutations affecting the first 120 amino acids at the amino-terminal end of the collagen type I triple helix had blue sclera but did not have dentinogenesis imperfecta. Among patients from different families sharing the same mutation, about 90 and 75% were concordant for dentinogenesis imperfecta and blue sclera, respectively. These data should be useful to predict disease phenotype in newly diagnosed OI patients.