Pulmonary lymphangioleiomyomatosis (LAM) is characterized by the proliferation of abnormal smooth muscle cells (LAM cells) and destruction of alveolar structure. Immunohistochemical studies suggest that excess matrix metalloproteinases (MMPs) synthesized by LAM cells function in the proteolytic mechanisms of this disease. We postulated MMP levels in the blood are elevated in LAM patients. Serum samples were collected from 36 LAM patients and 25 controls, and gelatinolytic activities were semi-quantified by gelatin zymography. The reliability of serum data for MMP-9 was confirmed by the measurement of MMP-9 concentration in plasma by enzyme-linked immunosorbent assay as well as by gelatin zymography. Serum levels of MMP-9 (0.7+/-0.1 AU), but not MMP-2, were significantly elevated in LAM patients compared with controls (0.1+/-0 AU). Plasma and serum levels of MMP-9 significantly correlated. These results suggest the involvement of MMP-9 in LAM.