The trends of the seasonal (April-September) and monthly reference evapotranspiration time series were analyzed using linear regression. In addition, the trends of air temperature and sunshine hours also were investigated to show possible causes of the increase in reference evapotranspiration. The spatial variability of reference evapotranspiration was lower than the temporal variability and is similar to the spatial distribution of precipitation in the area of Poland. The highest reference evapotranspiration was recorded in the region with the lowest rainfall. The statistically significant linear increasing trends of reference evapotranspiration were determined at each analyzed station in 1971-2010. The increase in the growing season sum of reference evapotranspiration, averaged over 18 stations, was 30 mm per 10 years. The significant increase in reference evapotranspiration can be explained by the statistically significant increasing trends of air temperature and sunshine hours.