A case of Legionella pneumophila infection complicated by thrombotic thrombocytopenic purpura (TTP) was confirmed at autopsy by the demonstration of the organism in lung tissue, and by the finding of widespread intravascular and subendothelial thrombi associated with microinfarctions in all major organs examined. In addition to the typical hematologic abnormalities of TTP, the patient was found to have a low serum C3 level and elevated levels of immune complexes as measured by the liquid phase C1q binding assay. We suggest that the L pneumophilia infection caused endothelial damage and/or platelet aggregation, perhaps as a consequence of complement activation, thus initiating the sequence of events leading to extensive microvascular thromboses.