Background: Porokeratosis is an uncommon disorder of epidermal keratinisation. Characteristic features include hyperkeratotic lesions with an atrophic centre and raised wall-like border, and the pathognomonic histological finding of a coronoid lamella.1 Malignant transformation into squamous, bowenoid or basal cell carcinoma rarely has been reported in all major variants.2 Aims: To describe a case of squamous cell carcinoma arising in porokeratosis of Mibelli. Methods: A 73-year-old man presented for routine skin malignancy surveillance with a new, asymptomatic lesion within a porokeratosis of Mibelli on his right calf, clinically suspicious for squamous cell carcinoma. His history included multiple non-melanoma skin cancers and one previous melanoma in situ. No other lesions typical for porokeratosis were found on examination. Histopathological examination of the lesion confirmed moderately differentiated squamous cell carcinoma arising in background changes of porokeratosis. The lesion was completely excised within a week of biopsy. Results and conclusions: Porokeratosis is a premalignant condition, however malignant transformation is rare.1 It is hypothesised that chromosomal instability and reduced immune surveillance with overexpression of p53 can contribute to the development of cutaneous malignancy within porokeratoses.2 Risk factors for malignancy include ionising radiation, sun exposure, immunosuppression, long history of the lesion and large size of the porokeratosis.3 This case adds to the existing literature surrounding malignant transformation of porokeratoses and highlights the importance of regular skin surveillance in affected patients.