BACKGROUND The importance of growth factors, such as platelet-derived growth factor (PDGF), for stromal activation in colorectal cancer is unclear. METHODS The expression of beta-receptors for PDGF, and PDGF B-chain (PDGF AB and PDGF BB) was investigated by immunohistologic techniques in full-thickness biopsies from 210 colorectal cancers. These antigens were detected by the monoclonal antibodies PDGFR-B2 and PDGF 007, respectively. RESULTS All tumors contained granular clusters of PDGF beta-receptor expressing stromal cells, whereas tumor epithelium was invariably negative. The staining was most prominent in vascular cells. There were several cells in the tumor stroma that expressed PDGF AB/BB. Double immunofluorescence stainings in specimens from four patients performed in order to characterize PDGF beta-receptor- and PDGF AB/BB expressing cells showed that cells expressing PDGF beta-receptors did not express PDGF AB/BB. About 20% of cells in the stroma expressing PDGF AB/BB were macrophages (CD68-positive cells), whereas the nature of the remaining stromal cells expressing PDGF AB/BB could not be disclosed. Furthermore, about 30% of CD68-positive macrophages expressed PDGF AB/BB, but not PDGF beta-receptors. The extent of clusters of PDGF beta-receptor expressing cells varied considerably between tumors, and its prognostic value was considered in the entire tumor material. The number of clusters did, however, not correlate to tumor differentiation, tumor stage according to Dukes', or outcome. CONCLUSIONS The presence of cells expressing PDGF beta-receptor and PDGF AB/BB respectively, i.e., expression of the receptor and its ligand, fulfills two of the prerequisites for a role of PDGF in the activation of stromal cells in colorectal cancers. The data suggest that stromal activation, characterized by clusters of PDGF beta-receptor expressing cells, is of importance for the formation of tumor stroma per se. However, the expression of the PDGF beta-receptor has no potential as a prognostic marker.