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Q Fever, caused by Coxiella burneti, was first identified as a separate syndrome in 1937 in Australia. Usually it takes the form of an acute atypic lung pneumonia disease, but subclinic or non-typical forms are also known. Q Fever is a zoonoses. The most important source of infection is represented by sheep; the transmission to man occurs by vectors (ticks), or more often by carriers as aerosol, non-pasteurized milk and dairy products. In Italy the first outbreaks of Q Fever date back to the end of World War II and lated until the beginning of the sixties. Since the seventies only a few sporadic cases have been notified, suggesting a low endemic situation. In Tuscany Q Fever presented the same epidemiological characteristics as in Italy. The causes of this particular epidemiological evolution are not clear. We suspect that it might be partly an illusion, caused by an inaccurate evaluation of the real number of cases, due to both the high frequency of atipic clinic forms and the lack of attention of doctors. On the other hand, a reduction in the circulation of C. burneti in the environment could be explained by a limitation of infection to man caused by changed sheep-breeding systems and a consequence of socio-economic evolution. To evaluate the actual reduction in the incidence of Q Fever in general, it is necessary: 1 - to make a careful research of Q Fever cases in man 2 - to evaluate the frequency of C. burneti antibodies in man and sheep. In this light, the Authors carried out a seroepidemiological study in individuals living in two different geographic areas of Tuscany. A total of 130 serum samples were tested. Half of them were collected from people professionally in contact with ovines, the other half from urban people presumably not in contact with animals. The antibody response to C. burneti was tested by indirect immunofluorescence test (IIT), a method known to be sensitive, specific, practical and economic. Antibodies titers indicating previous infection (greater than 1:20) were more frequently found in subjects belonging to the rural group (49.1%) than in those belonging to the urban group (6.1%). The seropositive subjects presented a negative clinic anamnesis. These results suggest a persistent circulation of C. burneti in man, linked to contacts with sheep. Also there appear, however, to be a good number of unnotified or subclinic Q Fever forms. Young people appear to be slightly less frequently infected, which suggest that the infection may be in fact becoming rarer.(ABSTRACT TRUNCATED AT 400 WORDS)