This invention provides a method for reducing the amount of osteopontin in an osteopontin-expressing cell comprising introducing into the cell a nucleic acid which specifically inhibits osteopontin expression in the cell. This invention also provides methods for inhibiting the onset of, and treating, osteopontin-related disorders, as well as compositions for practicing the same. This invention further provides methods for determining the amount of osteopontin in a sample, and a kit for practicing the same. This invention also provides methods for determining whether an agent reduces the amount of osteopontin in an osteopontin-expressing cell. Finally, this invention provides methods for treating a subject afflicted with a disorder mediated by an endogenous protein.