A 20-year-old woman had persistent erythematous spots on both thighs for three months. Examination showed symmetrical dark red reticular spots on the inner thighs with indistinct borders. The spots did not fade under pressure and were accompanied by slight local scaling. Last year, the patient worked at a desk and held a vacuum flask of hot water between her thighs. This is erythema ab igne, a skin condition caused by prolonged and repeated exposure to heat. Common heat sources include electric blankets, heating pads, hot water bottles, open fires and electronic devices. Heat is transferred by conduction, radiation or current, resulting in characteristic skin changes. The exact pathophysiology of erythema ab igne is not fully understood.