This paper examines the size and distribution of the Polish population in the Ukraine and the Ukrainian population in Poland. The first section discusses the historical setting that led Polish and Ukrainian populations to live together on the wide Polish-Ukrainian borderland. The paper then goes on to look at the scale and direction of migration movements that took place after World War II. As a result of these movements, comprising a total of about 2 million inhabitants, the new border between Poland and The Ukrainian Soviet Socialist Republic became a clear ethnic demarcation separating Poles from Ukrainians. The size of the Polish population in the Ukraine during the postwar period was examined on the basis of Soviet censuses carried out in 1959, 1970, 1979 and 1989. They showed a gradual reduction in the size of Polish population (in 1959 - 363.3 thousand, 1989 - 219.2 thousand). This decrease was caused by assimilation processes. Soviet census data were questioned. For this reason, the outcome of a Ukrainian census, conducted in new political conditions, was eagerly awaited. This census was conducted in 2001 and revealed only 144.1 thousand Poles, mostly living in Zytomierski and Chmielnicki region. The author gives reasons for the decline in the Polish population. The second part of the article discusses the situation of the Ukrainian population in Poland. Because dispersed, Ukrainians in Poland had difficulties in maintaining their national and religious distinction. Particular attention is paid to the consequences of the 'Vistula Action'. As a result of the action, strong processes of acculturation and assimilation of the Ukrainian community into the Polish environment occurred. The next section presents an outcome of the Polish census conducted in 2002. It revealed that 36.8 thousand Ukrainians live on Polish territory, of whom 5.9 thousand declared Lemko nationality. Most Ukrainians reside in the Warmia and Mazury province. In conclusion, the author compares the living conditions of Poles in the Ukraine and Ukrainians in Poland. He advances the thesis that both minorities will be subject to an intense process of assimilation.