Paederus dermatitis develops when beetles of the genus Paederus (often called rove beetles) are crushed on the skin, releasing the vesicant pederin. These beetles are found in many tropical and subtropical habitats. We describe 156 patients who presented to a dermatology clinic in the Guilan province of northern Iran during a 6-month period (May-October 2001). The peak time of presentation was in September, and the face and neck were the most common sites of involvement. Clinically, the most common presentation comprised geographic erythematous plaques with micropustules. In three-quarters of patients, more than one lesion was present. Kissing lesions were seen in 5% of cases, and 15% of patients developed diffuse desquamation. The majority of patients resided within 1 km of rice fields and used fluorescent lighting at home. In half of the cases, another family member was also affected. Paederus dermatitis is a common skin condition in northern Iran. We believe that increased public awareness of this condition can decrease mucocutaneous exposure to pederin.