The paper presents data on 32 alien plant species as well as seven ones native for other regions of the country which had not been recorded in NE Poland before or were known so far from only a few sites. The most important source of the spread of alien species in the initial stages of colonization is their intentional introduction into cultivation. Ornamental plants constitute 48.7% of recorded new species. The railway routes allowed spontaneous colonization of nine species (23%). The remaining species (28%) were most likely introduced by road transport of people and goods. The vast majority of alien species (77%) begin colonization in highly disturbed man-made habitats, especially ruderal ones in urban areas, as well as roadsides and embankments along communication routes, and so far they have appeared only in the sites of such types in the region. Only a few plants of alien origin are able to colonize forest habitats (i.a. trees and shrubs spreading by birds: Crataegus coccinea, Malus toringo and Rubus laciniatus). The expansion of Miscanthus sacchariflorus may pose the greatest risk of entering semi-natural habitats and naturally valuable areas in the near future. Predictions regarding the possibility of an explosive spread of Iva xanthiifolia did not come true. This may also apply to Erechtites hieraciifolius.