AIM: Squamous differentiation in eccrine porocarcinoma (EPC) is an unusual phenomenon that has rarely been reported in the literature. This study describes the clinical and pathological findings in a series of 21 cases of EPC showing extensive squamous differentiation. METHODS: The H&E-stained sections, epithelial membrane antigen and carcinoembryonic antigen immunohistochemical stains were reviewed for each case. The following variables were examined: age, gender, race, site and size of the EPC. The prevalence of other cutaneous lesions and/or underlying systemic disease was also documented. RESULTS: There was an almost equal gender distribution. Mean age was 61.5 years and the average tumour size was 46.5 mm. An inordinately large number (10/21, 48%) of EPCs occurred in black patients. The tumours were located at various sites with the extremities predominating (10/19, 53%). Seven patients developed other sun-induced skin tumours, three patients were renal transplant recipients, and two patients were HIV-positive, one of whom also suffered from albinism. Six of the 11 patients in whom follow-up was available had an adverse outcome: local recurrence developed in one patient, one patient developed nodal metastases, and one patient experienced both local recurrence and nodal metastases, and of the three patients who died of disease, two developed distant metastases. CONCLUSION: The findings suggest a possible role for ultraviolet radiation and chronic immunosuppression in the induction of malignant squamous differentiation in a subset of EPCs. Further reports on this histological variant of EPC are required to determine whether a pathogenetic link does indeed exist or whether these tumours simply represent a unique variant of squamous cell carcinoma with divergent acrosyringial differentiation.