Hairy leukoplakia is a recently described oral mucosal condition seen in immunosuppressed individuals, usually in association with HIV infection, when it is thought to be a sign of decreasing immunocompetence. It probably results from reactivation of infection by Epstein-Barr virus (EBV) and usually presents as bilateral white patches on the lateral borders of the tongue. From a histological study of 20 cases we have found that the typical appearance of hairy leukoplakia is of acanthotic, hyperparakeratinized epithelium with Candida hyphae sometimes present in the parakeratin. A band of EBV infected, koilocyte-like cells is present in the upper part of the prickle cell layers, these cells being swollen and pale staining, with prominent cell borders and perinuclear vacuoles. There is a paucity of inflammation in both the epithelium and lamina propria. An atypical appearance shows the koilocyte-like cells lying isolated or in small groups, irregularly arranged in the prickle cell layer and without a hyperparakeratinized surface. Diagnosis of hairy leukoplakia should normally be confirmed by demonstrating EBV in the koilocyte-like cells by immunocytochemistry or DNA in situ hybridization.