We encountered a patient with chronic myelogenous leukemia in basophilic crisis accompanied with histamine excess symptoms including bronchial asthma and gastric ulcer. The concentrations of histamine and histidine decarboxylase in leukemic cells containing granules typical for basophils were similar to those in mature basophils. His histamine excess symptoms rapidly disappeared concomitant with the reduction of blast cells after chemotherapy. We speculate that his histamine excess symptoms were induced by the leukemic cells.