Seborrhoeic keratosis (SK) is a benign epidermal tumour with a varying degree of pigmentation. We have recently demonstrated that endothelin-1 (ET-1) is a strong keratinocyte-derived mitogen and melanogen for human melanocytes in UVB-induced melanosis. To clarify the role of ET-1 in hyperpigmentation in SK, we have used immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR) to see whether the production of ET-1 is accentuated in SK. Immunohistochemical analysis in SK (n = 7; acanthotic and deeply pigmented types) revealed marked immunostaining with anti-ET-1 in almost all basaloid and basal cells as compared with definite staining confined to basal cells in the perilesional normal controls. In parallel, RT-PCR of ET-1 mRNA demonstrated accentuated expression of ET-1 transcript in SK (n = 4) in comparison with that in the perilesional normal controls, accompanied by a similarly accentuated expression of tyrosinase mRNA. These findings suggest that ET-1 plays a part in the hyperpigmentation seen in SK.