Trench fever is caused by Bartonella (Rochalimaea) quintana, a small gram-negative rod that is transmitted by body lice. Recently, B. quintana infections in homeless patients have been reported in the United States and Europe. From October 1993 to October 1994, the seroprevalence of antibodies to B. quintana was assessed by indirect immunofluorescence in a prospective study of 221 nonhospitalized homeless people, 43 hospitalized homeless patients (cases), 250 blood donors, and 57 hospitalized matched controls. Four (1.8%) of 221 nonhospitalized homeless people tested had titers of > 1:100. Of the 43 cases, seven (16%) had serological titers of > or = 1:100. None of the 250 serum samples from blood donors contained antibodies to B. quintana. The presence of antibodies to B. quintana in cases was significantly associated with the presence of body lice, exposure to cats, headaches, eastern European origin, and pain in the legs. This study demonstrates the presence of antibodies to B. quintana in the homeless population and should alert physicians that B. quintana might be an etiologic agent of fever in homeless patients.