Systemic mastocytosis associated with urticaria pigmentosa seems to be strikingly more common than previously assumed. The diagnosis can be established by the investigation of bone marrow sections, whereas bone marrow smears are less reliable. Some mediators are produced by the enhanced number of mast cells; telemethyl imidazole acetic acid is the most suitable mediator to calculate the size of the mast cell pool. Investigations like this might offer an alternative to the examination of bone marrow sections in future.