Here we describe a series of cases of spontaneous coagulopathy in a colony of inbred WAG/RijYcb (WAG/RijY) rats. This strain previously had been bred at our institution without symptomatology for several decades. The index case was a 10-wk-old male rat that developed a large hematoma at a subcutaneous injection site. Clinicopathologic findings included a decreased RBC count, decreased hematocrit, decreased hemoglobin concentration, normal PT, and prolonged (50% to 70%) aPTT (52 s; reference, 15 to 33 s). Examination of additional WAG/RijY rats that died unexpectedly or had clinical signs of bleeding in the absence of experimental manipulation also revealed normal PT and prolonged aPTT. Histologic examinations of tissues from all rats were unremarkable except for severe acute focally extensive hemorrhage corresponding to the macroscopic findings of acute hemorrhage. Furthermore the aPTT in 8 clinically normal adult rats and 8 clinically normal 4-wk-old WAG/RijY littermates of both sexes was prolonged. We conclude that these WAG/RijY rats have an inherited defect in the intrinsic coagulation pathway.