In this 16 year old boy a syndrome, characterized by high fever, generalized lymphadenopathy, splenomegaly, diffuse skin rash, facial and periorbital edema, neutropenia, thrombocytopenia, elevated serum glutamic oxaloacetic transaminase (SGOT) levels and transient electrocardiographic changes, appeared 2 weeks after the institution of diphenylhydantoin therapy. Lymph node biopsy, performed at the height of the illness, revealed widespread subendothelial fibrin exudation and fibrin-platelet thrombi in the lymph node microvasculature, a finding most consistent with thrombotic thrombocytopenic purpura. Although many types of abnormal lymph node histology have been described with diphenylhydantoin, this appears to be the first instance of this histologic picture. This syndrome may be related to a serum sickness-like illness which triggered an episode of localized coagulopathy.