AIM: To study the relationship between Chlamydia pneumoniae (Cpn) and spontaneous brain bleeding. METHODS: The IgG and IgM antibody titers to Cpn in peripheral blood from 9 patients of spontaneous brain bleeding were detected by microimmunofluorescence antibody technique (MIF). The Cpn DNA of arteries in brain bleeding region from these cases was detected by polymerase chain reaction (PCR). The Cpn major outer membrane protein (MOMP) expression of arteries in brain bleeding region from these cases were tested by Western blot. RESULTS: All the geometric average titer to IgG antibody (190 3?3 2) and the positive rate of IgG antibody (85 2%) and IgM antibody (30 8%) were significantly higher in spontaneous brain bleeding patients than those (48 1?2 0, 48 1% and 4 8%) in control ( P