The objectives of this study were twofold. One objective was to analyze the effects of growth in the social security tax, when combined with recent changes in U.S. income tax law, on the distribution of the combined income and social security tax burden during the 1980s. The second objective was to estimate the effects of certain proposals for social security tax reform upon that distribution. The above analyses were performed using simulation techniques applied to the 1984 IRS Individual Tax Model File. The data from this file were used to estimate the income and social security tax liabilities for sample taxpayers under tax law in effect in 1980, 1984 and 1988 and under fourteen proposals for social security reform (under 1988 law). The results indicated that the income tax distribution was almost 25 percent more progressive under 1988 tax law than under 1980 tax law. In contrast, the combined distribution of income and social security taxes was almost 25 percent less progressive under 1988 income and social security tax law relative to 1980. Two types of social security tax reform were analyzed. One type consisted of reforms to the basic social security tax structure, such as removal of the earnings ceiling, provision of exemptions and replacement of the current single tax rate with a two-tiered graduated rate structure. The second type of reform consisted of proposals to expand the theoretical tax base subject to the social security levy. The results suggested that these reforms could generate substantial increases in progressivity in the combined tax distribution. In general, it would appear that changes in the social security tax structure could generate greater improvements in progressivity than expansion of the theoretical tax base, although the greatest improvement was associated with a combination of these two reforms. With regard to horizontal equity, expansion of the theoretical tax base generated the most improvement.