A potentially contributing factor to the development and chronicity of pyoderma gangrenosum is infection with the relatively recently characterized human pathogen, Chlamydia pneumoniae. C pneumoniae is an obligate intracellular bacterium that can infect endothelial, monocyte, and smooth muscle cells and is associated with cardiopulmonary diseases. A case of serologically, polymerase chain reaction-positive, immunohistochemically, and culture-documented viable C pneumoniae organisms in a chronic pyoderma gangrenosum ulcer is reported, a finding that has not been described previously. (J Am Acad Dermatol 2003;48:966-9.)