The paper analyzes the economic impacts of a personal income tax reform in Poland. The reform would be based on increasing a tax-free threshold on personal income to 12 times the subsistence level of a one-person household. The results could be threefold: first, a decline in government income from personal income tax (PIT); second, an increase in government income from consumption tax (VAT); third, increased consumption triggering a rise in domestic production and generating additional government income. This could reduce the proportion of public debt to GDP. A short-term analysis of the first two impacts was performed by calculating the real level of private consumption tax, the number of PIT taxpayers, and total revenue from PIT. The post-Keynesian Stock-Flow Consistent model was employed to estimate long-term impacts. The analysis shows that the short-term effects of the reform would reduce government income by 3.1% while increasing the deficit by 19.1% (according to 2013 data). In the long term, the triggered rise in consumption and savings would spur GDP growth. This, in turn, would cause personal income to grow owing to multiplier effects. While the budget deficit would increase, the public debt-to-GDP ratio would fall. Thus the reform would not create the risk of surpassing the constitutional thresholds for this ratio.