In a retrospective study, the laboratory findings from the first blood samples taken following hospital presentation in patients with uncomplicated leptospirosis have been compared with the corresponding data for patients admitted, to a high-dependency medical ward or intensive-care unit, with severe leptospirosis. The aim was to identify those laboratory markers that differentiate the two clinical groups upon initial presentation. Marked differences were observed, in some of the haematological and clinical-chemistry markers, between the patients with severe leptospirosis and those with the uncomplicated disease. Statistically significant differences were found in haemoglobin concentrations, haematocrits, counts of erythrocytes, leucocytes, neutrophils and platelets, and serum concentrations of creatinine, urea, protein and albumin. These markers may therefore be useful in the assessment and early detection of disease severity in patients with suspected leptospirosis. Investigations into the use of albumin treatments, which might significantly improve the clinical care of patients with acute leptospirosis, appear to be justified.