['We describe an observational study in critically ill medical patients showing the association between serum Vitamin B12 levels measured on or near admission and the outcome in these patients.', 'We used the database of patients admitted to the Medical Intensive Care Unit (MICU) at the Hadassah-Hebrew University Medical Center in Jerusalem, Israel, to analyze associations between patient demographics, background, diagnoses and serum Vitamin B12 levels with hospital and 90 day outcomes.', 'Higher mean Vitamin B12 levels were found in patients who did not survive their hospital stay (1719\xa0pg/ml vs 1003\xa0pg/ml, p\xa0<\xa00.01). Those who had died by 90 days after admission to the MICU also had higher Vitamin B12 levels than survivors (1593\xa0pg/ml vs 990\xa0pg/ml). Regression analysis showed that elevated Vitamin B12 levels were associated with increased 90 day mortality, even after controlling for other variables. Survival analysis also showed an increased mortality rate in patients with Vitamin B12 levels over 900\xa0pg/ml (p\xa0<\xa00.0002).'] Are increased Vitamin B12 levels associated with mortality in critically ill medical patients?