Aim: Vitamin B12 (cobalamin) is nutrient from the vitamin B complex family. It is essential in the human body for deoxyribonucleic acid (DNA) synthesis and for cellular energy production. Vitamine B12 deficiency is decrease in its serum concentration below 220 pmol/L, and it can be present in various range of hematologic and systemic symptoms and signs. The aim of this article is to provide extensive information on extreme forms of deficiency of vitamin B12. Case report: 83-year-old men presented with fatigue, intolerance of physical activity, vertigo, paresthesia in fingers, dispersion, epigastric pain, vomitting and loss of apetite. Vital parameters were within normal range, and physical examination did not reveal any patological findings, except icterus of sclera and subicterus of skin. Urgent laboratory findings showed erythrocytes 1.18 x 1012/L (4.34-5.72 x 1012/L), hemoglobin 50 g/L (138-175 g/L), mean corpuscular volume (MCV) 123.6 fL (83.0-97.2 fL), vitamin B12 <61 pmol/L (220-665 pmol/L). Patient started parenteral therapy with vitamin B12. After 5 days, rapid increase in reticulocyte count was observed, with haemogram stabilization within 6 weeks. In a follow-up period of two years, the patient had no symptoms. Conclusion: We presented patient with extreme deficiency of vitamin B12. Screening for vitamin B12 deficiency should be established in each patient with macrocytic anemia, even with slightly elevated MCV.; Cilj: Vitamin B12 (kobalamin) je nutrijent iz porodice vitamina B-kompleksa. U ljudskom organizmu esencijalan je za sintezu deoksiribonukleinske kiseline (DNK) i proizvodnju energije u stanicama. Deficijencija vitamina B12 jest pad njegove koncentracije u serumu ispod 220 pmol/L, a može se prezentirati u širokom rasponu hematoloških i sistemskih simptoma i znakova. Cilj ovog članka je pružiti ekstenzivnu informativnost o ekstremnim oblicima deficijencije vitamina B12. Prikaz slučaja: Pacijent u dobi od 83 godine javio se zbog iznemoglosti, intolerancije napora, vrtoglavica, trnaca u prstima, dekoncentracije, bola u epigastriju, povraćanja i gubitka apetita. Bio je normalnih vitalnih parametara, a fizikalnim pregledom nije pronađeno patološkog nalaza, osim žutice. Hitni laboratorijski nalazi bili su: eritrociti 1.18 x 1012/L (4.34 – 5.72 x 1012/L), hemoglobin 50 g/L (138 – 175 g/L), prosječni volumen eritrocita (MCV) 123.6 fL (83.0 – 97.2 fL), vitamin B12 <61 pmol/L (220 – 665 pmol/L). Započeta je parenteralna terapija s vitaminom B12. Nakon 5 dana zapažen je rapidan rast retikulocita, uz stabilizaciju hemograma u roku od 6 tjedana. U kontrolnom razdoblju od dvije godine nakon liječenja pacijent nije imao simptoma. Zaključak: Prikazali smo pacijenta s ekstremnom deficijencijom vitamina B12. Probir na deficijenciju vitamina B12 trebao bi biti proveden kod svakog pacijenta s makrocitnom anemijom, čak i s blago povišenim MCV-om.