Proteoglycans are macromolecules displaying structural roles as well as regulatory functions in the maintenance of the extracellular matrix. Biglycan/PG-I and decorin/PG-II are two small proteoglycans that are structurally related but differ considerably in their localization in vivo and behaviour in vitro. Decorin and, to a minor extent, biglycan, can be located at the surface of type I collagen fibrils and have been shown to influence collagen fibrillogenesis. However, the physiological role of biglycan in the dermis is not known. Biopsies obtained from keloids were bisected and processed for total RNA extraction and immunohistochemistry. Northern blot analysis of total RNA obtained from keloids with high growth tendency in vivo showed a marked induction of biglycan and collagen alpha 1(I)mRNA expression in comparison with total RNA obtained from normal skin or keloids with little growth tendency. In contrast, decorin mRNA expression remained largely unaltered. Studying these biopsies by immunohistochemistry, decorin expression in the dermis was unaltered comparing normal and keloid tissue, whereas a markedly increased staining for biglycan was observed in the keloid tissue, which was most pronounced in the nodular formations, and was a characteristic feature of keloids. The altered expression of biglycan in keloid tissue might be involved in the abnormal regulation of extracellular matrix deposition either through the binding of growth factors or by influencing the three-dimensional organization of collagen fibres or associated molecules.