Background: Tissue repair often occurs in organs damaged by various inflammatory diseases including pneumonia. Inflammatory stimuli induce a rapid and massive release of inflammatory cells from the bone marrow. Recent studies have suggested that bone marrow cells have the potential to differentiate into a variety of cell types. It has been shown that administration of lipopolysaccharide (LPS) to murine lungs induces a rapid release of endothelial progenitor cells (EPCs) into the circulation, and that bone marrow derived progenitor cells including EPCs contribute to lung repair after lung injury in mice. This study was undertaken to investigate the mobilisation of EPCs in humans following acute pneumonia. Methods: Peripheral blood mononuclear cells (PBMCs) were isolated from venous blood taken from 23 patients with pneumonia during both the acute and convalescent phase. 1×106 PBMCs were plated on fibronectin coated culture slides and cultured in culture medium for endothelium. The numbers of EPCs were counted 8 days after plating. Results: The number of circulating EPCs significantly increased in patients with pneumonia (p<0.0001). Patients with low EPC counts tended to have persistent fibrotic changes in their lungs even after their recovery from pneumonia. Conclusions: Inflammatory stimuli induce a rapid release of EPCs into the circulation in humans. A sufficient number of EPCs is necessary for proper lung repair following bacterial pneumonia.