Congenital osteogenesis imperfecta (OI) was diagnosed by ultrasound in a 31-week-old fetus, and the diagnosis confirmed after delivery by caesarean section at week 36. The baby survived the neonatal period, but failed to thrive, had recurrent respiratory infections and ultimately died at 8 months. Cultured fibroblasts synthesized both normal type I collagen and unstable type I collagen harbouring a structural defect in the alpha 1 (I) cyanogen bromide-derived peptide number 8 (CB8) region of the molecule, indicating a heterozygous dominant mutation. At birth, the radiological picture was that of the "thin bone"-type of congenital OI (OI type IIB/III in the Sillence classification); at the age of 12 weeks ribs and long bones had undergone a marked expansion giving a very different picture, that of the "thick bone"-type congenital OI (OI type IIA). The mechanism responsible for this change in bone structure is not known, but fractures and callus formation are unlikely to be the only factors. Caution is needed in the interpretation of radiographs of newborns with OI for prognostic or genetic purposes.