Basaloid proliferations overlying dermatofibromas which morphologically resemble superficial basal cell carcinomas have been interpreted as both reactive/regressive and frankly malignant. Metallothioneins (MTs) are low-molecular-weight proteins with a selective binding affinity for heavy metal ions. MTs has been proposed to represent a biological marker of carcinogenesis and, in a variety of human tumours, a correlation between immunohistochemically overexpression of MT and aggressive clinical behaviour has been shown. In order to clarify the nature of basaloid proliferations overlying dermatofibromas, we examined, immunohistochemically, 10 dermatofibromas with overlying simple hyperplasia, 16 dermatofibromas with overlying basaloid proliferation, and 35 basal cell carcinomas, for expression of MT. In normal epidermis, the basal keratinocytes showed cytoplasmatic MT immunoreactivity. The staining intensity was stronger in the basal cells of the rete ridges, an observation which is in accordance with the high proportion of S-phase cells in this area. Simple hyperplasia showed the same MT expression pattern as normal epidermis. Basaloid proliferations stained like superficial and nodular basal cell carcinomas. Of nodular basal cell carcinomas, 92% (12 of 13) showed decreased/absent MT immunoreactivity, while 86% (six of seven) of infiltrating/morphoea-like basal cell carcinomas showed overexpression of MT (P = 0.001, Fisher's exact test). The results demonstrate that MT overexpression in basal cell carcinomas is correlated with infiltrative growth pattern. The similar expression of MT in basaloid proliferations and 'non-infiltrating' basal cell carcinomas suggests that these lesions share a common change in metabolism and/or differentiation.