Objectve: Vtamn B12 defcency s a common health ssue n chldren. Though the sensvty of serum vtamn B12 measurement for the dagnoss of vtamn B12 defcency s low, combned methylmalonc acd, homocystene and holotranscobalamn have been reported to be more specfc bomarkers n early and accurate dagnoss. In ths study, we amed to verfy the dagnoss of vtamn B12 defcency n chldren by measurng methylmalonc acd, homocystene, holotranscobalamn and to determne the accuracy of them. Materal and Methods: The study ncluded 277 patents aged <18 years old age and wth a vtamn B12 levels <200 pg/ml. The cut-off values for defnng vtamn B12 defcency were methylmalonc acd>0.4 μmol L, holotranscobalamn <21 pmol/L. Homocystene levels were evaluated accordng to age. Results: There was a postve correlaton (kappa ndex 0.387) between vtamn B12 and holotranscobalamn and a negatve correlaton between vtamn B12, methylmalonc acd and homocystene levels (kappa ndex -0.134, -0.258, respectvely). There was a weak correlaton between methylmalonc acd and holotranscobalamn; methylmalonc acd and homocystene; homocystene and holotranscobalamn (kappa ndex -0.039, 0.243, -0.377, respectvely). Patents were dvded nto 4 groups accordng to holotranscobalamn, homocystene and methylmalonc acd levels. Seventy-three patents (26%) had low holotranscobalamn. Group 1a was consstng of 37 patents who were dagnosed vtamn B12 defcency wth low serum B12 and supported by all 3 parameters. When all groups were compared, vtamn B12 and hemoglobn levels were found to be lower n Group 1a than other groups (respectvely, p<0.05, p<0.001) Concluson: In chldren, holotranscobalamn alone should not be consdered an accurate ndcator, and other functonal markers should be combned n the dagnoss of vtamn B12 defcency. Furthermore, most studes about vtamn B12 defcency were carred out n adult age groups, and studes are needed n chldren to determne the reference ntervals for holotranscobalamn and serum methylmalonc acd.