The mechanism of water-induced pruritus in patients with polycythemia vera is unknown. Evidence has been presented previously that bathing or showering may trigger mast cell degranulation and that release of a mediator by mast cells may be responsible for the pruritus. Tryptase is a specific marker of human mast cell secretory granules and its presence in body fluids indicates mast cell degranulation. In this study, serum tryptase levels were measured both before and one hour after showering in 11 patients suffering from polycythemia vera and water-induced pruritus. Tryptase was not found in the serum of any of the subjects one hour after showering, when levels would be expected to be near peak had significant mast cell degranulation occurred. These results argue against mass cell degranulation-with systemic release of a mast cell product as the mechanism for water-induced pruritus in patients with polycythemia vera.