Introduction. Osteogenesis imperfecta (OI) is etiologically heterogeneous disorder characterized by childhood osteoporosis. A subtype OI type V is caused by the same c.-14C>T mutation in the IFITM5 gene. Nevertheless, there is a marked interindividual phenotypic variability in clinical presentation; however, response to bisphosphonates is reported to be good. Methods. Two individuals with OI type V had multiple recurrent fractures with hypertrophic calluses, scoliosis and ossifications of the forearm interosseous membranes. Sequencing of IFITM5, genotyping of variants rs2297480 in farnesyl diphosphate synthase gene (FDPS), and rs3840452 in geranylgeranyl diphosphate synthase 1 gene (GGPS1), both involved in bisphosphonate metabolism, was performed. Results. In patient 1 BMD reached normal values during bisphosphonate treatment and remained normal four years after the treatment discontinuation. In patient 2 no increase in BMD after five years of bisphosphonate treatment was observed and callus formation continued. The c.-14C>T IFITM5 mutation in heterozygous state was detected in both individuals. Additionally, both patients carried FDPS variant rs2297480 in homozygous state, and were heterozygous for GGPS 1 variant rs3840452. Conclusions. The paper presents a short overview of childhood osteoporosis with a special emphasis on OI type V by presenting two cases. Both OI type V patients had identical disease-causing mutation, but marked interindividual phenotypic variability. The striking failure in response to bisphosphonate treatment in one of the patients could not be explained by the variants in genes involved in bisphosphonate metabolism. Uvod. Osteogenesis imperfecta (OI) je vzročno heterogena bolezen, katere značilnost je osteoporoza v otroštvu. Pri vseh opisanih bolnikih s podtipom OI tipa V je vzrok bolezni ista mutacija c.-14C>T gena IFITM5. Kljub temu med bolniki obstaja izrazita fenotipska variabilnost v klinični sliki, toda opisan je le dober odgovor na zdravljenje z bisfosfonati. Metode. Oba bolnika z OI tipa V sta imela ponavljajoče se zlome kosti s hipertrofičnimi kalusi, skoliozo in zakostenelo membrano med podlahtnico in koželjnico. Opravili smo sekvenčno analizo gena IFITM5 in genotipizacijo variant rs2297480 gena za farnesil difosfat sintazo (FDPS) in rs3840452 gena za geranilgeranil difosfat sintazo 1 (GGPS1), ki sta vpletena v presnovo bisfosfonatov. Rezultati. Pri bolniku 1 se je ob zdravljenju z bisfosfonati mineralna kostna gostota povečala do normalnih vrednosti in ostala nespremenjena štiri leta po prenehanju zdravljenja. Pri bolniku 2 kljub pet let trajajočemu zdravljenju z bisfosfonati ni prišlo do izboljšanja mineralne kostne gostote, še naprej so se pojavljali zlomi kosti in hipertrofični kalusi. Pri obeh bolnikih smo ugotovili znano mutacijo c.-14C>T v genu IFITM5 v heterozigotni obliki. Oba imata v homozigotni obliki prisotno tudi varianto rs2297480 gena FDPS in v heterozigotni obliki varianto rs3840452 gena GGPS1. Zaključek. V članku je predstavljen kratek pregled osteoporoze v otroštvu s posebnim poudarkom na OI tipa V pri dveh bolnikih. Pri obeh bolnikih z OI tipa V, ki sta imela različno klinično sliko in potek bolezni, smo ugotovili mutacijo c.-14C>T v genu IFITM5. Z analizo genov encimov, vpletenih v presnovo bisfosfonatov, nismo mogli pojasniti neuspešnega zdravljenja z bisfosfonati pri enem od bolnikov.