The aim of the undertaken research was to assess the regional diversification in economic effectiveness of private enterprises in Poland. The assessment was conducted at NUTS II regional level encompassing the voivodships for two selected years - 1999 and 2006. The studies confirmed that regional diversification is indicated by all three indicators of the economic effectiveness of enterprises, i.e. labor productivity, gross return on capital employed and return on equity. However the scale of that diversification measured by the variability coefficient level decreased during the period covered. For example, in 1999 the variability coefficient for the ROE was as much as 75.5%, which means high diversity of voivodships as, concerns the level of that indicator. On the other hand, in 2006 that indicator was at the level of 18,9% and the voivodships proved less diversified as concerns that indicator.