A 53-year-old man developed lichenoid lesions on the upper chest, posterior surfaces of the trunk, and abdominal region about three months before his first visit. Physical examination and laboratory findings were normal; histopathology showed vacuolar degeneration of basal keratinocytes in association with a dense lympho-histioid infiltrate arranged in a lichenoid pattern with a few melanophages and eosinophils. The fact that our patient had been irregularly taking sildenafil citratus (Viagra) led to the hypothesis of a lichenoid drug-induced eruption. Our hypothesis was confirmed by clinical resolution three weeks after discontinuation of sildenafil citratus; moreover, the patient avoided the drug for about four months, and the eruption didn't reappear. Subsequently, we performed a challenge test with the drug, and the patient developed similar lichenoid lesions. Lichenoid eruptions are rather common dermatoses that can be induced by a great number of environmental agents and are clinically but not pathogenetically well defined. We report the present case because, despite the great number of drugs that can be implicated in the development of lichenoid eruptions, the association of such dermatoses and sildenafil citratus had been described only once previously in the literature until now. Furthermore, we wish to remark on the significance of a detailed anamnestic history to make the correct differential diagnosis between lichenoid drug-induced eruptions and lichen planus. This has a great clinical importance because simple discontinuation or substitution of the drug causes lichenoid drug-induced eruption resolution.