A patient with laboratory-acquired Rocky Mountain spotted fever (RMSF) and concomitant influenza virus infection was studied from the third day of clinical illness. The course of his illness was marked by petechial rash, thrombocytopenia, and elevation of fibrin split products. No evidence of complement activation was observed. Plasma proteins were elevated in a pattern characteristic of the "acute phase reaction." The patient recovered completely, and vascular collapse or clinically important disseminated intravascular coagulation had developed. In febrile patients who had influenza or a clinically similar noninfluenzal respiratory syndrome, no changes in coagulation, complement, or plasma proteins developed. We conclude that aberrations in the patient's laboratory values reflected RMSF, and that complement played no critical role in his illness.