In this paper we develop a measure of competition based on management's disclosures in their 10-K filing and find that firms’ rates of diminishing marginal returns on new and existing investment vary significantly with our measure. We show that these firm-level disclosures are related to existing industry-level measures of disclosure (e.g., Herfindahl index), but capture something distinctly new. In particular, we show that the measure has both across-industry variation and within-industry variation, and each is related to the firm's future rates of diminishing marginal returns. As such, our measure is a useful complement to existing measures of competition. We present a battery of specification tests designed to explore the boundaries of our measure and how it varies with the definition of industry and the presence of other measures of competition.