Obstructive sleep apnoea syndrome (OSAS) is a very common disorder. Patients with OSAS are at an increased risk for cardiovascular events. It has also been reported that a 25% rise in factor VII clotting activity (FVIIc) is associated with a 55% increase in ischaemic heart disease death during the first 5 years. We examined the effects of nasal continuous positive airway pressure (NCPAP) treatment on FVIIc in patients with OSAS. FVIIc was investigated prospectively in 15 patients with OSAS before (mean +/- SEM apnoea and hypopnoea index (AHI) 61.5 +/- 4.2 and after (AHI 3.0 +/- 0.9) NCPAP treatment for immediate relief, at 1 month after treatment and at over 6 months. FVIIc levels gradually decreased after NCPAP treatment. After 6 months of NCPAP treatment, FVIIc levels had decreased significantly (before 141.1 +/- 11.7% vs. after 6 months 110.7 +/- 6.2%; p < 0.01). Six of the seven patients whose FVIIc levels were over 140% before the NCPAP treatment had FVIIc levels below 130% after 6 months or 1 year of NCPAP treatment. This decrease in FVIIc after long-term NCPAP treatment could improve mortality in OSAS patients. If patients, especially obese ones, present with high FVIIc of unknown origin, it would be prudent to check for OSAS.