The hematopoietic cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) is being used in clinical trials for its potential in the treatment of hematopoietic insufficiency due to various causes. Involvement of leukotrienes in the effects of GM-CSF is suggested by analytical and pharmacologic evidence obtained in vitro. However, until now no data in support of a role of leukotrienes in GM-CSF action in vivo have been presented. In the present investigation this question was approached by measurement of endogenous cysteinyl leukotriene formation in patients treated with the cytokine for cytopenia induced by cytostatic drugs or for refractory anemia with excess of blasts (RAEB). Endogenous cysteinyl leukotriene formation was assessed by determination of urinary leukotriene metabolites using combined high- performance liquid chromatography and radioimmunoassay analysis. After GM-CSF administration a distinct increase in urinary cysteinyl leukotrienes was found in the cytopenic and the RAEB patients that ranged from 2.3- to 57-fold and 2.4- to 333-fold, respectively. In the cytopenic patients the increase in leukotriene production was correlated to an expansion of peripheral blood leukocytes; RAEB patients responded to GM-CSF with enhanced leukotriene biosynthesis even if the peripheral leukocytes decreased, possibly due to an abnormal number and/or irritability of leukotriene-producing cells. The increase in endogenous leukotriene production during therapy with GM- CSF may indicate that leukotrienes play a role in GM-CSF action in vivo.