Aims: To determine the relevance of lymphopenia to the diagnosis of bacteraemia in patients admitted with medical emergencies, relative to peripheral blood white cell count and neutrophilia. Patients/Methods: A two year cohort study carried out in a teaching hospital in Oxford, UK of 21 495 consecutive adult emergency admissions to general medical or infectious disease wards. Full blood data were available in 21 372 cases; 41 cases with extreme full blood count results (neutrophil count, > 75 × 109/litre; lymphocyte count, > 10 × 109/litre) were excluded, leaving 21 331 cases for analysis. The association between the admission lymphocyte and neutrophil counts and the risk of bacteraemia was assessed. Results: Neutrophilia and lymphopenia were both associated with bacteraemia. Lymphopenia was the better predictor in this cohort. Both neutrophilia and lymphopenia were more predictive of bacteraemia than the total white blood cell count. Conclusions: Both lymphocyte and neutrophil counts, rather than total white blood cell count, should be considered in adult medical admissions with suspected bacteraemia.