IL-1120-ST Instructions
Illinois Department of Revenue - Illinois Small Business Corporation Replacement Tax Return (includes Schedules B and NB) You must file Form IL-1120-ST if you are a small business corporation, as defined in Internal Revenue Code (IRC), Section 1361(a), that has net income or loss as defined under the Illinois Income Tax Act (IITA); or is qualified to do business in the state of Illinois and is required to file U.S. Form 1120S (regardless of net income or loss).