It has been suggested that the involution of the pigmented lesions of halo naevus (HN) is mediated by an immune response, with the involvement of specific cytotoxic T lymphocytes. To explore further the pathogenesis of HN, skin biopsies from six patients with this condition were obtained and the characteristics of the infiltrating inflammatory cells were studied by immunostaining techniques. We found that the cell infiltrate of HN is mainly composed by CD8+ T lymphocytes that express the activation molecule CD69. Tumour necrosis factor-alpha (TNF-alpha) immunoreactivity was detected on the inflammatory cells, a finding that suggests that the infiltrating T cells of HN are actively synthesizing this cytokine. Our results indicate that the infiltrating cells of HN predominantly have an activated cytotoxic phenotype, and suggest that these cells are indeed involved in the regression of the naevomelanocytic naevus of HN.