• OI Type I
• OI Type II
• OI Type III
• OI Type IV

Type III is distinguished among the other classifications as being the "Progressive Deforming" type, wherein a neonate presents with mild symptoms at birth and develops the aforementioned symptoms throughout life. Lifespan may be normal, albeit with severe physical handicapping.

Type III is the second most severe form of the disease. OI Type III can affect as many as one in sixty thousand people. OI Type III bones fracture very easily a many fractures may occur within the womb of the mother. Fractures often present at birth, and x-rays may reveal healed fractures that occurred before birth. Many bones may also be broken during the birthing process. The collagen in the bones of Type III individuals is improperly formed. Loose joints and poor muscle development in their arms and legs making it impossible for them to ever accomplish walking.

It is compatible with longer life, although people are generally of below-average height, may have skeletal and/or respiratory problems, and brittle teeth.

Type III OI affects 20 percent of the people who have OI. Usually the result of a spontaneous mutation, it is common for a person with this type to have experienced 100 fractures by the time he or she reaches puberty.

Some features of Type III Osteogenesis Imperfecta include:

• Short stature (some people only grow three feet tall).
• Sclera has a blue, purple, or gray tint.
• Soft bones that not only break easily but also bend.
• They have major spinal curvature causing chronic back pain and respiratory problems.
• They will always use a wheelchair for mobility throughout their entire life span.
• Their organs, which are the same size as any normal person’s, are restricted within the smaller rib cage and because of this they experience problems with their organs.
• As with other types of OI, they have a triangular shaped face.
• Poor muscle development.
• Poor tooth development, often causing teeth to be brittle and discolored.
• Possible hearing loss.
• Possible respiratory problems.
• Loose joints and poor muscle development in arms and legs.