The main objective of the study was to determine changes in the prices of live lamb and lamb carcasses in Poland and to indicate the occurring regularities. The specific objectives were to show the changes in the prices of live lamb and lamb carcasses after the accession to the EU, to identify the periods with the highest and the lowest prices and to define the occurring regularities in this respect. The sources of materials are data from EUROSTAT, the Central Statistical Office (CSO) and the Regional Union of Sheep and Goat Breeders in Białystok. The research period covered the years 2004-2020. Descriptive, tabular and graphical methods were used to analyse and present the materials, as well as fixed-base dynamics indicators. In Poland, prices of livestock and lamb carcasses did not change significantly during the several years after EU accession. There were short periods of price decreases and increases. Seasonal price fluctuations were also found. The highest prices for lamb carcasses were achieved from December to Easter, while the lowest prices were achieved during the summer months. The reason for this was the seasonality of both production and consumption. There were also regional disparities in the purchase prices of lamb livestock due to differences in sheep production and use.