The expression of tumour necrosis factor (TNF)-alpha in leprosy skin lesions was examined before and during successful treatment in a patient with borderline lepromatous leprosy. Before treatment, immunohistochemical staining of a skin biopsy specimen showed diffuse TNF-alpha deposits in granulomas and significant TNF-alpha deposits on infiltrated mononuclear cells. After 1 year's treatment, the skin lesions exhibited a reduction in granulomas, and a concomitant reduction in deposits of TNF-alpha. Furthermore, the level of expression of TNF-alpha messenger RNA, as examined using a reverse transcriptase-polymerase chain reaction method, was reduced markedly after treatment. These findings provide evidence for a correlation between the expression of TNF-alpha and disease activity suggesting that TNF-alpha is a useful prognostic indicator for inflammation in leprosy.