||The number of zoonotic Bartonella species identified in the last 15 years has increased considerably, since the first HIV- infected patient with unusual vascular proliferative lesions of bacillary angiomatosis (BA) was described in 1983. Of the 21 species of Bartonella described until now, only 10 were acknowledged as human pathogen species. B. bacilliformis, B. quintana, and B. henselae are the most frequently described species, while B. elizabethae, B. vinsonii, B. washoensis, B. grahamii, B. clarridgeiae, B. koehlerae and B. alsatica were recently identified as responsible for a few cases of human infections. Cat scratch disease (CSD), bacillar angiomatosis, hepatic peliosis and some cases of bacteraemia, endocarditis, osteomyelitis, uveitis and neurologic disorders are directly caused by some species of the genus Bartonella. We realize three studies, the objective of cats study, the first aimed to evaluate the prevalence of Bartonella infections in Spain, was to know the seroprevalence against B. henselae in cats, the level of bacteremia and to assess the factors that could modify both prevalences. Since cats are the main reservoir of Bartonella, we have studied the infection by B. henselae in a sample of cats in our geographical area. We conducted microbiologic cultures and serology to a total of 115 cats, 33. 05% of which were younger than one year. Of the 100 cultures performed (15 animals were rejected due to massive contamination), we isolated Bartonella in 7, with a prevalence of bacteriemic cats below one year of 12. 12%. Isolated strains were identified as B. henselae by PCR, using the 16S-23S rRNA intergene region primers. Thirty-four cats (29. 56%) showed antibodies against B. henselae with titers above 1/64. Of all the factors related to the bacteraemia rate studied (age, sex, habitat, contact with animals, parasites, and presence of antibodies against Bartonella) only the presence of antibodies against Bartonella henselae was statistically significant. To determine the real incidence of Bartonella infections, we must study the seroprevalence in the general population as well as the principal reservoirs and vectors of infection transmission. Serum samples from 218 patients were examined by indirect immunofluorescence assay (IFA). Significance levels for univariate statistical test were determined by the Mann-Whitney U test and χ2 test. Of 218 patients, 99 were women and 119 men, with a median age of 34. 36 years (range 0--91 years). Nineteen (8. 7%) reacted with B. henselae antigens. Of all the factors concerning the seroprevalence rate being studied (age, sex, contact with animals, residential area), only age was statistically significant. Our serological data seems to indicate that Bartonella henselae is present in our area and could be transmitted to humans. – And in the last study, serum samples were collected from HIV patients treated at Hospital de Sabadell. Antibodies to B. henselae and B. quintana from 340 patients were examined by indirect immunofluorescence assay (IFA). Significance levels for univariate statistical test were determined by the Mann-Whitney U test and χ2 test. – Of 340 patients, 82 were women and 258 men, with a median age of 42. 21 ± 10. 35 years (range 16 -86 years). Seventy-six (22. 3%) patients reacted with one or more Bartonella antigens. Of all the factors concerning the seroprevalence rate being studied (age, sex, intravenous drugs use, alcohol consumption, CD4 levels, AIDS, HCV, HBV, residential area), only age was statistically significant. A high percentage of HIV patients presents antibodies to Bartonella and is increasing with age.