Hyaluronan (HA) is one of the extracellular-matrix components involved in wound healing, tumour growth and metastasis. Due to the limited data on HA expression in benign and malignant breast lesions, we analyzed its presence in these lesions by using the biotinylated-hyaluronan-binding region and the link-protein complex (bHABC) of cartilage proteoglycan as a specific probe. In all benign breast lesions, the expression of HA was restricted to the stromal connective tissue, the ductal epithelial cells being completely devoid of HA. In malignant breast tumours, the intensity of stromal HA staining was significantly stronger than in benign lesions. In addition, HA was detected on cell membranes or in cytoplasms of adenocarcinoma cells, in some cases of ductal carcinoma in situ and in 31% of malignant tumours. The staining pattern was mostly similar in all breast-cancer types studied, i.e., ductal, lobular, tubular, mucinous and medullary. In ductal breast cancer, intense HA expression in stroma and carcinoma cells correlated statistically significantly to poor differentiation of carcinoma, suggesting that altered HA expression may affect the mechanisms of breast-cancer progression.