A proband with osteogenesis imperfecta (OI) type III/IV was born to clinically normal parents, who subsequently had two pregnancies terminated because of OI in the fetuses. Cultured fibroblasts from the proband, one fetus and the father produced abnormal collagen I. Cyanogen bromide mapping localised the defect to the region of the alpha 1(I)CB7 peptide. Sequencing revealed a G to A transition at nucleotide 2814 in COL1A2 in the proband, the fetus, and the father, which resulted in a Gly802Asp substitution in the pro alpha 2(I) collagen chain. About 25% of the paternal alleles from fibroblasts and leucocytes and 40% of paternal alleles from spermatocytes carried the mutation consistent with somatic and germinal mosaicism. For genetic counselling, parental mosaicism must be considered in all sporadic cases of OI.