During the last three years we performed field trials to assess levels of resistance against potato virus X (PVX) and changes in intrinsic properties of the potato cultivars Bintje and Escort upon the introduction of the PVX coat protein (CP) gene. Analysis of leaf and tuber samples collected in the field at two week intervals revealed a stable expression of the PVX CP gene throughout the growing season. This resulted in a large decrease in PVX incidence among clonal progeny obtained from previously infected Bintje and Escort clones. Based on evaluation of 50 defined morphological characteristics, tuber yield and grading, 81.8% of the Escort and 17.9% of the Bintje derived transgenic clones proved to be true to type. Overall lightsprout morphology was a useful criterion for the early detection of deviant transgenic clones. Using the polymerase chain reaction (PCR) with convergent primers spanning transgenic sequences, true to type clones could be distinguished unambiguously from the corresponding untransformed cultivars. Clear distinctions between independent transgenic clones could be made by inverted PCR (IPCR) diagnosis revealing integration-specific border fragments. These results demonstrate the commercial feasibility of improving potato cultivars by selectively adding new traits while preserving intrinsic properties, and the possibility of unambiguously identifying independent transgenic cultivars.