Masses of the prepuce and penis are common clinical presentations for the equine veterinarian, encompassing a number of differential diagnoses. In such cases, it is important to evaluate the external genitalia thoroughly and devise a methodical diagnostic and treatment plan to allow for accurate prognostication and optimal survival rates. Masses of the prepuce and penis include tumours of epithelial, mesenchymal or round cell origin, such as squamous cell carcinoma (SCC), papillomas, melanocytic tumours, lymphoma, sarcoids, fibrosarcomas (Van Den Top et al. 2010) and fibromas; the latter as described by De Meyer et al. in this issue (De Meyer et al. 2015). Non-neoplastic conditions that can result in masses or enlargement of the external genitalia in male horses include infection or inflammation (primary, or secondary to neoplasia) leading to balanitis or balanoposthitis. Such diseases frequently lead to chronic irritation and discomfort. Some neoplastic conditions may metastasise to cause more severe sequelae.