The effects of elevated fibrinogen on thrombin and reptilase times have not been well documented. High fibrinogen levels are common (38% of specimens submitted to our coagulation laboratory). Among 102 patients in the present study, an endogenously elevated fibrinogen level was significantly associated, as follows, with prolonged reptilase times: 1 (4%) of 28 with normal fibrinogen levels, 6 (20%) of 30 with levels in the 400 to 700 mg/dL (4.0-7.0 g/L) range, 10 (34%) of 29 with levels in the 700 to 1,000 mg/dL (7.0-10.0 g/L) range, and 7 (47%) of 15 with fibrinogen levels greater than 1,000 mg/dL (10.0 g/L). This association was independent of patient age and fibrin degradation product titer. In contrast, thrombin time was not altered notably by elevated fibrinogen levels. In 4 patients studied further, the prolonged clotting times could be corrected or nearly corrected by adding calcium chloride or albumin, whereas no such corrections were demonstrable in samples from several hereditary dysfibrinogenemia control subjects. An elevated fibrinogen level is common and is associated with reptilase time prolongations. For patients with prolonged reptilase times, a fibrinogen assay is suggested before establishing a diagnosis of dysfibrinogenemia.