We investigated haemostatic and inflammatory parameters in patients with cystic fibrosis in an attempt to understand a previous finding of low factor XII levels in this patient population. We selected two groups of patients, adults attending outpatient annual review clinic who were well, chronically inflammed and adult patients with an infective exacerbation requiring antibiotics or admission to hospital, acutely inflammed. We measured known positive acute phase haemostatic factors, fibrinogen and factor VIII. Antithrombin and factor XII were also measured as both these factors have been proposed to be negative acute phase proteins in in-vitro cell models. Interleukin-6 was also measured as the proposed modulator of these factors during inflammation. Activated factor XII was measured to exclude XII activation as a cause of the low XII activity levels. Cystic fibrosis patients admitted to hospital with infective exacerbations, showed significantly more evidence of inflammation than the annual review patients. Fibrinogen and factor VIII were higher and factor XII was lower in these patients. This work suggests that factor XII behaves as a negative acute phase protein with no signs of elevated activated XII levels in either group. This supports similar findings from in-vitro cell culture. This study also shows low antithrombin levels in both patient populations, although there was no statistical difference between groups, which is probably related to their liver disorder.