In 30 fully investigated family pedigrees in which there were at least two generations of people suffering from osteogenesis imperfecta type I (McKusick no. 16620), the data on 144 random offspring could be used for segregation analysis. The major characteristics, blue sclerae, fractures, and hearing loss, were present in every pedigree. Their penetrance was also calculated. Precise definitions were used in the study. The segregation ratio or observed: expected ratio was 70:72. The incidence of blue sclerae was 70:70 (100%), for fractures 61:70 (87%), and for hearing loss 30:70 (43%). There was a very clear relationship between age and the progression of the hearing loss. Dividing the offspring into two groups depending on whether or not male-to-male inheritance was present and performing segregation and penetrance calculation on these data did not produce any indications that there are two genetically distinguishable subtypes of osteogenesis imperfecta type I. In a smaller group of 107 offspring, calculations could be made on several separate generations.