This study investigates firms’ decision to withhold the identity of their major customers. I first document that the extent of private firms in the industry (private firm intensity) relates positively to non-disclosure of major customer identity. I next determine whether these results are motivated by competitive cost or agency cost concerns. Consistent with competitive cost concerns, the relation between private firm intensity and non-disclosure increases when operations involving major customer relationships are highly profitable. In addition, consistent with agency cost concerns, the positive relation between private firm intensity and non-disclosure increases when operations involving major customers are highly unprofitable. While disclosure of customer identity is mandated by the SEC, firms with unprofitable customer relationships appear better able to conceal these identities (i.e., to avoid disclosure requirements) by using the excuse of competitive harm from private firm intensity. As further evidence of agency cost, non-disclosure is increasing when major customers are better able to extract rents and when managers have low ability. As a final test, I find that non-disclosure of customer identity delays investors’ ability to assess the impact of customer distress on the supplier’s firm value. Results from this study demonstrate the important role that private firm intensity and customer relationship profitability jointly play in corporate disclosure decisions.