AbstractLeukotrienes (LT) exert stimulatory effects on myelopoiesis, beside their inflammatory and immunomodulating effects. Here, we have studied the expression and activity of the enzymes involved in the synthesis of leukotriene B4(LTB4) in acute myeloid leukemia (AML) cells (16 clones) and G‐CSF mobilized peripheral blood CD34+cells. CD34+cells from patients with non‐myeloid malignancies expressed cytosolic phospholipase A2(cPLA2), 5‐lipoxygenase activating protein (FLAP), and leukotriene A4(LTA4) hydrolase but not 5‐lipoxygenase (5‐LO). The enzyme cPLA2was abundantly expressed in AML cells and the activity of the enzyme was high in certain AML clones. The expression of 5‐LO, FLAP, and LTA4hydrolase in AML clones was in general lower than in healthy donor polymorphonuclear leukocytes (PMNL). The calcium ionophore A23187‐induced release of [14C] arachidonic acid (AA) in AML cells was low, compared with PMNL, and did not correlate with the expression of cPLA2protein. Biosynthesis of LTB4, upon calcium ionophore A23187 activation, was only observed in five of the investigated AML clones and only three of the most differentiated clones produced similar amounts of LTB4as PMNL. The capacity of various cell clones to produce LTs could neither be explained by the difference in [1 − 14C] AA release nor 5‐LO expression. Taken together, these results indicate that LT synthesis is under development during early myelopoiesis and the capacity to produce LTs is gained upon maturation. High expression of cPLA2in AML suggests a putative role of this enzyme in the pathophysiology of this disease.