A woman in her 50s presented with a 4-day history of a diffuse, progressive, and pruritic rash. She had no known allergies and did not take any medications. She had no exposures to chemicals, detergents, or new personal care products; however, the patient consumed raw shiitake mushrooms several days before symptom onset. An examination revealed dozens of long flagellate streaks composed of red papules distributed across the trunk and extremities (Figure). She denied fever, malaise, or dyspnea. Laboratory results were remarkable for eosinophilia. The findings and history were consistent with flagellate mushroom dermatitis. The patient was treated with topical steroids, oral antihistamines, and raw shiitake mushroom avoidance, and the condition improved without sequelae.