Fiberoptic bronchoscopy and bronchoalveolar lavage are often followed by the development of fever and other influenza-like symptoms. We report the onset of these symptoms in a healthy volunteer following bronchoscopy and bronchoalveolar lavage; the symptoms were temporally associated with the dramatic elevation of serum levels of tumor necrosis factor. Our observations suggest that tumor necrosis factor may be involved in mediating fever and other influenza-like symptoms occurring after bronchoscopy.