Osteopontin (OPN), an integrin-binding, transformation-associated protein, is secreted by tumor cell lines in culture and is associated with increased malignancy in some experimental tumor systems. Little is known, however, about the significance of OPN expression in human cancers. The aims of this study were to determine if OPN was expressed in a series of surgically resected lung cancers, and if there was a relationship between OPN expression and clinico-pathologic findings or outcome. Twenty-five patients who underwent curative pulmonary resection were studied prospectively. RNA was extracted from primary tumor and distant normal lung tissue for each patient. OPN RNA levels were evaluated by northern blotting. Immunohistochemistry on paraffin-embedded tissue, using an anti-OPN monoclonal antibody, was performed to assess tissue distribution of OPN protein. OPN RNA and protein were over-expressed in the majority of tumors, relative to paired normal tissue. There was variation in the cells of the tumor that were OPN-immunopositive. In some cases OPN was present in tumor cells, while in the majority of cases OPN was detected primarily in tumor-infiltrating macrophages and necrotic areas. Over-expression of OPN RNA or protein generally was not related to clinico-pathological findings. However, there was a statistically significant association between OPN-immunopositivity in the tumor and patient survival. These findings suggest that OPN levels in lung tumors have the potential to provide clinically important predictive information on patient outcome, and that OPN may play a role in the biology of lung cancer.