We studied blood histamine activity (HA) and cutaneous fibrinolytic activity (CFA) in a patient with polycythaemia rubra vera (PRV) and water-induced pruritus, before and after water exposure. The results suggest that the water-induced itching in PRV is associated with an increase in HA. In addition, markedly increased levels of CFA were found both before and after water exposure. These findings have been previously reported in patients with aquagenic pruritus (AP) but not in patients with PRV. As the water-induced itching in PRV and AP share many common features, these findings suggest that the pathophysiology of the water-induced pruritus in these two conditions may be similar.