Familial urticaria pigmentosa associated with thrombocytosis as the initial symptom of systemic mastocytosis and Down's syndrome.

Most cases of urticaria pigmentosa are confined to the skin, but visceral involvement and/or haematological abnormalities have been observed. It is still a matter of debate whether all forms of mastocytosis are true neoplasias or reactive hyperplasias. Familial inheritance of urticaria pigmentosa is rare.