The aim of the paper is to analyse the causal relationship between income inequality and economic growth in Poland in the period 1990 - 2006. The research method used is based on a study of literature on macroeconomics and international finance, as well as on econometric techniques. All statistics included in the paper are derived from the World Income Inequality Database and World Economic Outlook databases. The obtained results confirm that there is a significant causal relationship between income inequality and economic growth in Poland. Moreover, the study outcomes provide evidence of the existence of a non-linear relationship between the degree of income inequality and level of economic development. This relationship can be graphically represented as a curve shaped like an inverted letter 'U', which is consistent with the Kuznets hypothesis.