We report a cystic fibrosis (CF) subject with extensive, central venous catheter-associated thrombosis and sustained elevation of factor VIII to levels normally associated with significantly increased risks of deep venous thrombosis. To determine the potential significance of this finding, the prevalence of elevated factor VIII levels in 22 adults with CF was investigated. Mean (S.D.) factor VIII level was 177 (43) U/dl, with 77% of subjects having levels >150 U/dl. The high prevalence of elevated factor VIII levels questions the significance of this finding in CF subjects with catheter-related thrombosis.