Sweet cherry is a tall, deciduous tree producing stone fruits. This diploid outcrossing species was domesticated in Asia but has been grown in orchards and home gardens in Scandinavia for many years. In situ and ex situ assessments of phenotypic variation in sweet cherry accessions were performed to determine the reliability of such assessments, and to determine relationships between Nordic populations. Principal component analysis (PCA) based on in situ data revealed that accessions were mostly clustered according to their country of origin. PCA based on ex situ assessment of accessions that were propagated by seed at Hornun (Denmark) did not agree with the PCA based on in situ data. These contrasting results suggest that phenotypic assessment in sweet cherry depends on the environment, genotype, and the interaction between them. Phenotypic diversity accounted for by in situ assessment may not be always true, while phenotypic differences determined by ex situ assessment may be confounded by the genotype-by-environment interaction, or could depend on the new genotypes arising from open pollination after seed propagation. Our research also suggests that ecotype differentiation could occur in wild Scandinavian sweet cherry. Fruit descriptors were among the best to distinguish between Scandinavian populations. Previously reported monogenic characteristics showed intermediate narrow-sense heritability, as suggested by the percentage of total variation accounted by the half-sib populations. (C) 2000 Elsevier Science B.V.