Chlamydophila pneumoniae DNA was investigated by polymerase chain reaction (PCR), in cerebrospinal fluid (CSF) specimens from patients suffering from HIV-1-associated dementia complex (HADC). Four (17.3%) cases of C. pneumoniae infection were identified among 23 HADC individuals with DNA amplification of major outer membrane protein (MOMP) gene and 16S rRNA gene sequences. Sequence analysis revealed significant homologies with C. pneumoniae compared to Chlamydia trachomatis and Chlamydia psittaci. High mean levels of CSF specific anti-C. pneumoniae antibodies and C. pneumoniae antibody specific index values significantly elevated were also found by enzyme-linked immunosorbent assay (ELISA) in these patients. The results suggest a hypothetical role of C. pneumoniae in the pathogenesis or progression of HADC.