Taxing Income from Mailing List and Affinity Card Arrangements: A Proposal considers nonprofit organizations and the exception to their general tax-exempt status, the unrelated business income tax. While qualifying nonprofit organizations are exempt from most federal taxation, Congress never provided an explanation for the exemption. In order to fill this gap, several authors have provided their own rationales for the tax exemption. The author reviews the major rationales for permitting a federal tax exemption to nonprofit organizations and finds the unifying basis for all non-economic theories is the increased public trust and confidence in nonprofit organizations as compared to for-profit institutions. Therefore, the effect of a nonprofit's income stream on public trust should determine what income is exempt from the unrelated business income tax. This unifying rationale is then applied to two sources of nonprofit income - the rental or exchange of mailing lists, and affinity card programs. Because of their different effects on public trust, the author suggests that mailing list income should be taxable, but the income from affinity cards should remain exempt from taxation. This is the exact opposite of the position currently taken by Congress.