Two cases of systemic mastocytosis with different clinical course were described. The first of them manifested with returning attacks of tachycardia with raise of blood pressure, followed by its drop and loss of consciousness. Abdominal pain and persistent diarrhea characterized the second one. The common signs in both cases were skin changes of urticaria pigmentosa, presence of Darier's symptom, very high blood levels of tryptase and prostaglandin D2 and rise in urinary LTE4 concentration. Differential diagnostic is discussed and histopathology bone marrow biopsies are presented.