Subungual keratoacanthoma is a rare and uncommon variant of keratoacanthoma. It shows a locally aggressive behavior, and spontaneous regression is infrequently. The tumour usually occurs on the fifth decade of male caucasian patients, with predilection for the first three fingers of the hand. The cause is still unknown, and occurs more frequently in patients with incontinentia pigmenti and is considered a late manifestation of the disease. We report a patient's case with incontinentia pigmenti, who has a lesion in her right thumb, compatible with a subungual keratoacanthoma. This tumour is known for being difficult to diagnose, so it must be based on correlation of clinical, radiological and histopathological findings. It is crucial to make the differential diagnosis with subungual invasive squamous cell carcinoma, as they differ in treatment, since the latter has an aggressive behavior and needs radical treatment, while the handling of the keratoacanthoma might be conservative.