Syndecan-1 (SDC-1), a protein found on cells and in the extracellular matrix, participates in cell proliferation, cell migration and cell-matrix interactions. SDC-1 expression correlates with the maintenance of epithelial morphology and inhibition of invasiveness. In the present study, a second SDC-1 mRNA isoform was identified and the expression of both transcripts was investigated in various normal and malignant tissues. Both transcripts were coexpressed at equal levels in all tissues and organs analysed. Cancer-profiling array (CPA) analysis of 241 non-enriched tumour and normal cDNAs revealed stronger upregulation of SDC-1 in tumour tissues as compared with oligonucleotide array-based expression analysis of SDC-1 in microdissected breast, prostate, lung, and colon carcinoma cells. With in situ hybridisation and immunohistochemistry it was demonstrated that this difference in SDC-1 expression originates from stromal cells present in tumour connective tissue. But only the cells in connective tissue surrounding breast, lung, colon and bladder carcinoma showed upregulation of SDC-1. These stromal cells were characterised as spindle cells with myofibroblastic differentiation and they may contribute to the dedifferentiation of tumour cells and the development of metastasis.