Pro forma Form 1120
Definition
A Pro forma Form 1120 is a partially completed IRS corporate tax return used solely as a "cover sheet." For foreign-owned single-member LLCs, the IRS requires that Form 5472 be attached to this Pro forma Form 1120 when filed. "Pro forma" simply means you are only filling out the top identifying section of the form and leaving the complex financial sections blank.
Why it matters
When a non-US resident owns a single-member LLC, they must file Form 5472. However, the IRS systems are built to process Form 5472 only when it is attached to a corporate tax return (Form 1120). By filing a Pro forma Form 1120 with "Foreign-Owned U.S. DE" written across the top, you give the IRS the cover sheet they need to accept and process your Form 5472 without actually submitting to corporate taxation.
Example
Sofia is a non-US resident who owns a Delaware LLC. By April 15, she prepares Form 5472 to report her transactions. She then prints out Form 1120, writes "Foreign-Owned U.S. DE" at the top, fills out her LLC's name, address, and EIN, and leaves the rest of the form completely blank. She staples Form 5472 behind the Pro forma Form 1120 and faxes it to the IRS.