BACKGROUND: Chlamydia pneumoniae causes pneumonia and upper respiratory tract infection and has been recently reported to be associated with coronary atherosclerosis. The difference between C. pneumoniae and other Chlamydia spp. has been demonstrated by serologic study, DNA analysis and ultrastructural observation. However, studies concerning the developmental cycle of C. pneumoniae are relatively short. This study was conducted to investigate the morphological changes and developmental characteristics of C pneumoniae in the HeLa cell. METHODS: To observe the intracellular inclusion of C. pneumoniae, the cultured HeLa cell monolayer was stained with Jones' iodine and Giemsa. The ultrastructures were examined with an electron microscope at 6, 24, 48, 72 and 96 hr after inoculation of elementary bodies. RESULTS: The C. pneumoniae organisms which formed intracytoplasmic inclusion bodies in HeLa cells were negative on iodine stain. In Giemsa-stained preparation, the inclusion bodies of variable sizes with a bluish purple color were identified in the cytoplasm of infected HeLa cells. After 6 hrs of infection, the elementary bodies with electron-dense spicule shaped substance of C. pneumoniae were enclosed by the HeLa cell membrane and were taken the host cell by endocytosis. After 24 hrs of infection, the electron-dense material in the elementary bodies were disappearing and the elementary bodies were transforming into reticulate bodies. After 48 hrs of infection, the reticulate bodies of C. pneumoniae were seen dividing by binary fission. Small electron-dense round bodies(miniature bodies) appeared near completion of division. After 72 hrs of infection. about half of the reticulate bodies were transformed into elementary bodies. Newly formed elementary bodies had a pear-shaped structure and large periplasmic space. After 96 hrs of infection. mature elementary bodies with condensed electron-dense material and a rigid outer membrane were observed. Miniature bodies were located in the cytoplasm of the elementary bodies. CONCLUSIONS: These unique morphological changes in HeLa cell culture show the developmental characteristics of C. pneumoniae.