Objective: To evaluate the medical relevance of hyperhomocysteinemia in women with primary recurrent miscarriages.Design: Case report and retrospective cross-sectional study.Setting: Hematology outpatient department of a university hospital.Patient(s): Case report concerning a woman with five consecutive fetal losses. One hundred consecutive women with primary recurrent unexplained miscarriages (study group) and matched healthy controls (control group) with no antecedent fetal loss.Intervention(s): Venous blood sample collection in resting individuals.Main Outcome Measure(s): Plasma total homocysteine concentrations, plasma folate concentrations, and DNA analysis for the C677T mutation of the 5,10 methylene tetrahydrofolate reductase gene. Normal threshold homocysteine concentration was obtained from values found in the control group (95th percentile).Result(s): The case patient was hyperhomocysteinemic, was homozygous for the C677T mutation in the methylene tetrahydrofolate reductase gene, and had plasma folate deficiency. Folic acid and pyridoxine administration normalized the homocysteine concentration and favored a successful pregnancy. In the retrospective study, 12 of 100 patients were hyperhomocysteinemic. Twenty percent had the C677T methylene tetrahydrofolate reductase genotype and 15% had low plasma folate concentrations. The highest values of homocysteine concentration were found in patients with both the C677T genotype and folate deficiency.Conclusion(s): Hyperhomocysteinemia should be identified in women with recurrent miscarriages because therapeutic normalization might permit a normal birth.