Recessive forms of Osteogenesis Imperfecta (OI) may be caused by mutations in LEPRE1, encoding prolyl 3-hydroxylase-1 (P3H1) or in CRTAP, encoding cartilage-associated protein. These proteins constitute together with cyclophilin B (CyPB) the prolyl 3-hydroxylation complex that hydroxylates the Pro986 residue in both the type I and type II collagen α1-chains. We screened LEPRE1, CRTAP and PPIB (encoding CyPB) in a European/Middle Eastern cohort of 20 lethal/severe OI patients without a type I collagen mutation. Four novel homozygous and compound heterozygous mutations were identified in LEPRE1 in 4 probands. Two probands survived the neonatal period, including one patient, who is the eldest reported patient (177/12 y) so far with P3H1 deficiency. At birth, clinical and radiologic features were hardly distinguishable from those in patients with autosomal dominant (AD) severe/lethal OI. Follow-up data reveal that the longer-lived patients develop a severe osteochondrodysplasia that overlaps with, but has some distinctive features from AD OI. A new splice site mutation was identified in two of the four probands, affecting only one of three LEPRE1 mRNA splice forms, detected in this study. The affected splice form encodes a 736 amino acid (AA) protein with a ″KDEL″ endoplasmic reticulum retention signal. While western blotting and immunocytochemical analysis of fibroblast cultures revealed absence of this P3H1 protein, mass spectrometry and SDS-urea-PAGE data showed severe reduction of α1(I)Pro986 3-hydroxylation and overmodification of type I (pro)collagen chains in skin fibroblasts of the patients. These findings suggest that the 3-hydroxylation function of P3H1 is restricted to the 736AA splice form.