Advertising is a critical competition device that affects interactions among firms in the product market. I find that a nontrivial portion of public firms, even among those with intense advertising activities, do not disclose advertising expenses in their financial statements, indicating significant disclosure discretion. I further use product category-level data to construct measures that capture firm-level advertising rivalry. I predict and find that the likelihood of disclosing advertising expenses is negatively associated with advertising rivalry. This association is more pronounced when firms have less trackable media outlets, more volatile underlying advertising expenditures, and more mature products. These findings suggest that firms consider their advertising expenditures proprietary and that concerns of product market competition discourage their disclosure of advertising expenses despite the materiality constraint.