such as fixed drug eruptions, fixed solar urticaria, ultraviolet recall and radiation recall dermatitis.Mechanism of RU seems to be elucidative. A group of easily releasable mast cells accumulate at the site of allergen injections. In addition, distant injection of allergen or the systemic release of 'histamine-releasing factors' might precipitate degranulation of 'over-responsive' mast cells. 1 However, we could not explain the exact causes in our patients.