ObjectiveWe sought to assess the current magnitude of the opportunity for secondary stroke prevention with B vitamins.DesignA cohort study.SettingThe Urgent TIA (Transient Ischaemic Attack) Clinic at an academic medical centre.Main outcome measuresWe assessed the prevalence of biochemical vitamin B12deficiency (B12Def, serum B12<156 pmol/L), hyperhomocysteinaemia (HHcy; plasma total homocysteine [tHcy] >14 µmol/L) and metabolic B12deficiency (MetB12Def, serum B12<258 pmol/L and HHcy) between 2002 and 2017, by age group and by stroke subtype.ResultsData were available in 4055 patients. B12Def was present in 8.2% of patients overall; it declined from 10.9% of patients referred before 2009 to 5.4% thereafter (p=0.0001). MetB12Def was present in 10.6% of patients, and HHcy was present in 19.1% of patients. Among the patients aged ≥80 years, MetB12Def was present in 18.1% and HHcy in 35%. Among the 3410 patients whose stroke subtype was determined, HHcy was present in 18.4% of patients: 23.3% of large artery atherosclerosis, 18.1% of cardioembolic, 16.3% of small vessel disease, 10.8% of other unusual aetiologies and 13.6% of undetermined subtypes (p=0.0001).ConclusionsDespite a decline in our referral area since 2009, B12Def, MetB12Def and HHcy remain common in patients with stroke/TIA. Because these conditions are easily treated and have serious consequences, all patients with stroke/TIA should have their serum B12and tHcy measured.