Objective: The aim of this study is to evaluate the properties of vitamin B12 deficiency and predisposing factors in the patients of an outpatient clinic. Material and Methods: Four hundred patients with vitamin B12 deficiency were studied. Anthropometric measures, biochemical analysis, gastric parietal cell antibodies and parasites in stool of the patients, antigliadin antibody survey and upper gastrointestinal system endoscopy were performed. Additionally, randomly selected 100 patients with low and 100 healthy subjects with normal serum vitamin B12 levels were questioned about food intake. Results: Overall ratio of vitamin B12 deficiency among outpatients was 4%. Coexisting diseases such as diabetes mellitus (12.3%), hypertension (42.0%), hyperlipidemia (42.0%) and obesity (42.0%) were determined. Nearly a quarter (22,8%) had autoimmune thyroid disease. A group of patients had other problems that may have caused malabsorption such as Helicobacter pylori gastritis (66/85= 77.6%), antigastric parietal cell antibody positivity (37/85= 43.5%), antigliadin antibody positivity (40/61= 65.6%), and others (3.3%). The patient group with vitamin B12 deficiency consumed significantly less meat (p< 0.001), chicken (p< 0.001), fish (p= 0.002), milk (p= 0.027) and eggs (p< 0.001), smaller number of meals (p= 0.013) and significantly higher amounts of fiber (p< 0.001) and vegetables (p= 0.037) when compared with the control group. Conclusion: Vitamin B12 deficiency was considered to be related mainly with improper and defective alimentation. However, malabsorption-related diseases may also contribute to vitamin B12 deficiency.