Space-occupying processes in the eyelids can either be due to lesions which are specific for the eyelids, such as a chalazion or a meibomian carcinoma, or to non-specific lesions of the skin or ocular adnexa. An incorrect interpretation on clinical or histological grounds is often made; in other words, some eyelid processes masquerade as others. The correct diagnosis should be made on the basis of case history, localisation, the appearance of the tumour and the histological findings, so that adequate treatment can be given. One must be aware of the danger of mistaking one space-occupying process for another. In the Leiden Ophthalmological Clinic a study was made of the incidence of eyelid tumours in the period 1981-1987, with special attention to the most important masquerade phenomena observed. In a group of 956 eyelid tumours histological examination was necessary in 126 cases. Basal-cell carcinoma was the most frequent diagnosis, but other less common tumours such as a trichilemmoma, an eosinophilic granuloma, a haemangio-endothelioma, a metastasis from a mammary carcinoma and a granuloma annulare were also diagnosed.