To resolve uncertainty concerning the inheritance of the perinatal lethal form of osteogenesis imperfecta (OI type II), we collected family data and radiographs for 71 probands and analyzed the collagens synthesized by dermal fibroblastic cells cultured from 43 of the probands, 19 parental pairs, and single parents of each of four additional probands. In 65 families for which there were complete data on sibship size, there was recurrence of the OI type II phenotype in five families such that six (8.6%) of 70 sibs were affected. In two families with recurrence, the radiographic phenotype was milder than that for the remainder; and one of those families was consanguinous, suggesting autosomal recessive inheritance. In the remaining three families there was no evidence of consanguinity, but in one of them the structure was compatible with gonadal mosaicism in the mother. In studies of collagens synthesized by cells from 43 infants, we identified two probands with separate rearrangements in an allele of one of the genes of type I collagen; but in the rest there were subtle mutations that disrupted the normal triple-helix structure of type I collagen molecules. In two probands we identified de novo mutations; in 16 additional families cells from the parents made only normal collagens, compatible with new mutations in their offsprings. These findings indicate that the OI type II phenotype is biochemically heterogeneous, that the majority result from new dominant mutations in the genes encoding type I collagen, and that some recurrences can be accounted for by gonadal mosaicism in one of the parents.