Obstructive sleep apnoea (OSA) is associated with increased cardiovascular morbidity and mortality. Several studies indicate an influence of OSA on haematological features. There is evidence of elevated platelet activation and increased haematocrit in OSA. Studies investigating the underlying cause of polycythaemia in OSA reveal different results: increased haematocrit in OSA might be due to enhanced erythropoietin excretion and/or plasma volume dysregulation based on altered release of volume-regulating hormones such as atrial natriuretic peptide (ANP). This review addresses haematological issues associated with OSA, especially platelet function, haematocrit and haemoconcentration. 2001 Harcourt Publishers Ltd