Type I procollagen is a heterotrimer composed of two proα1(I) chains and one proα2(I) chain, encoded by the COL1A1 and COL1A2 genes, respectively. Mutations in these genes usually lead to dominantly inherited forms of osteogenesis imperfecta (OI) by altering the triple helical domains, but a few affect sequences in the proα1(I) C-terminal propeptide (C-propeptide), and one, which has a phenotype only in homozygotes, alters the proα2(I) C-propeptide. Here we describe four dominant mutations in the COL1A2 gene that alter sequences of the proα2(I) C-propeptide in individuals with clinical features of a milder form of the disease, OI type IV. Three of the four appear to interfere with disulfide bonds that stabilize the C-propeptide conformation and its interaction with other chains in the trimer. Cultured cells synthesized proα2(I) chains that were slow to assemble with proα1(I) chains to form heterotrimers and that were retained intracellularly. Some alterations led to the uncharacteristic formation of proα1(I) homotrimers. These findings show that the C-propeptide of proα2(I), like that of the proα1(I) C-propeptide, is essential for efficient assembly of type I procollagen heterotrimers. The milder OI phenotypes likely reflect a diminished amount of normal type I procollagen, small populations of overmodified heterotrimers, and proα1(I) homotrimers that are compatible with normal skeletal growth.