Although red cell transfusion is a relatively common treatment in medically ill patients, much is not known about the clinical determinants used to guide its use. We conducted a retrospective, case-controlled chart review of admissions to the medical teaching units (MTU) at this center (Queen Elizabeth II Health Sciences Centre, Halifax, Nova Scotia). Forty-two patients who received red blood cell transfusion during admission to the MTU from January to March 31, 2004. Clinical data collected included: age, gender, indication for MTU admission, co-morbid disease, smoking, admission hemoglobin, pre- and post-transfusion hemoglobin, arterial pressure of oxygen, lactic acidosis, parameters of shock, presence of active bleeding, and need for surgery. Logistic regression analysis was used to identify significant clinical determinants of transfusion. Red blood cell transfusion rate was 8%. Of admission diagnoses and co-morbidities, only cancer was associated with a trend for transfusion (p=0.07). Those who received red blood cell transfusion had significantly lower admission hemoglobin levels (p=0.005), higher serum creatinine (p=0.02), and lower mean arterial pressure (p=0.01). Significant predictive clinical factors of red blood cell transfusion included congestive heart failure (OR=3.72, CI 1.034, 27.160), admission hemoglobin level (d/L, OR=0.956, CI 0.927, 0.986), mean arterial pressure (mm Hg, OR=0.942, p=0.005) and serum creatinine (ml/min, OR=1.008, CI 1.003, 1.014). In conclusion, predictors of red blood cell transfusion for medically ill patients include low admission hemoglobin, renal insufficiency, and low blood pressure.