The paper studies the spatial aspects of the 20-year demographic history of the Karelian Isthmus based on a wide range of demographic statistics for the Soviet Union and Finland (including unpublished data). Different data on the population dynamics of the whole region and separate parts of it are given. It is shown that for the isthmus in 1920–1939, moderate growth rates of the population are characteristic, which lag slightly behind the average rates across Finland. The demographic development of the isthmus differed by a pronounced irregularity: the population in the territories adjacent to the cities and rural industrial centers increased at the highest rates. By 1939, about 295000 people lived in territory belonging to Finland and which at present is included in the structure of Leningrad oblast; a medium population density was characteristic, with an increase to a high density in the suburbs of cities and industrial centers. Within the short period of 1940–1941, the prewar demographic potential of the territories attached to the Soviet Union recovered by approximately half, but primarily due to the urban population. The development of the space of the isthmus decreased sharply during this period. Under the Finnish occupation of the isthmus in 1941–1944, the prewar demographic potential and peculiarities of its spatial organization were significantly restored, but only for a short period. The paper specifies the main spatial differences of the results of the second colonization (1944–1949) of the Karelian Isthmus from the first one (1940–1941) and their reasons. Special attention is devoted to the characteristics of urbanization in the newly developed territory; the relationship of these processes with the consequences of the war is described. In addition, the role of the altered economic and geographic position of the transferred territories during transformation processes of the spatial organization processes of the population is shown.