ABSTRACT Introduction: Vitamin B12 deficiency is emerging as a growing public health problem in obstetrics. A number of studies show vitamin B12 deficiency to be a risk factor in female infertility. Conventionally hormonal assay has been used in the investigation of infertility; this is without recourse to vitamin B12 assay. This study therefore aims at estimating vitamin B12 level in infertile women attending Obstetrics and Gynecology clinic in Nnamdi Azikiwe University Teaching Hospital (NAUTH) Nnewi with the view of determining its role or relationship in female fertility in the study subjects. Method: A total of 165 subjects with different fertility status who were in the child bearing age were recruited into this cross sectional and comparative study through simple random sampling. The enlisted subjects were then grouped into 4 based on their fertility status: group A (35) had primary infertility, group B (22) had secondary infertility, group C (28) had recurrent miscarriages and group D (80) were non pregnant fertile women as control. Fasting blood (5mls) was collected, 2mls was dispensed into EDTA bottles for FBC determination using Procan PE 6800 analyzer and the remaining 3mls dispensed into plain container, the serum was used for batch vitamin B12 estimation by ELISA method. Results: Vitamin B12 (pg/ml) level of the infertile subjects was 232.15±241.13 while the control 406.29±214.58. Comparison using t-test, a statistical difference of (p < 0.05) was observed. The Vitamin B12 for each of the 4 groups based on their fertility status using ANOVA test statistics: group A was 174.33±156.94; group B had 461.84±277.25, while group C and D were 157.15±214.91 and 406.29±214.58 respectively. The Post Hoc intra group comparison of the 4 groups reveals a statistical difference between groups A (10 infertility) & B (20 infertility), A & D(control), B & C(recurrent abortion) and C &D (p < 0.05). No difference was observed for the RBC, PCV, Hb levels between the test and control groups. The case is same for platelet counts, total WBC and its differentials except in neutrophil count. Conclusion: This study suggests that lower level in vitamin B12 may be associated with infertility particularly in primary and recurrent abortion cases in subjects studied.