We report three cases of eccrine poroma, present for many years, in which features were seen suggesting transformation from a benign to a malignant tumour. These changes ranged from in situ Bowenoid dysplasia to frankly invasive squamous carcinoma. The most helpful diagnostic feature in distinguishing the origin of the tumours was the presence of strong cytoplasmic staining for carcinoembryonic antigen (CEA) in cells surrounding, and giving rise to, neoplastic ducts and clefts. Dermatopathologists examining eccrine poromata should examine the lesions carefully for any evidence of malignant change.