In cancer patients with primary tumor with or without metastasis or metastasis alone, by comparison with healthy subjects and noncancer patients, the decrease in blood histamine levels is due to a decrease in total basophil number. These basophils have a normal content of histamine (1 to 2 pg per basophil) and are able to release histamine. The percentage of anti IgE-induced histamine release is not significantly different than in noncancer patients. The scarcity of basophils in cancer patients is not due to a leukopenia. Blood histamine levels and total basophil number are normal in patients after successful excision of their primary tumor without metastasis.