The expression of leukocyte alkaline phosphatase (LAP) in neutrophils is reduced in some patients with myelodysplastic syndrome (MDS). We quantitatively assayed for LAP in MDS leukocytes by a flow cytometry based method using a monoclonal antibody raised against human bone alkaline phosphatase. The LAP expression was assayed in blood samples from a group of 46 MDS patients, consisting of 39 patients with refractory anemia (RA), 3 with RA with excess blasts (RAEB), and 4 patients with RAEB in transformation. The percentage of LAP-positive cells was significantly higher in the MDS patients than in the normal subjects and also higher in RA than in RAEB and RAEB in transformation. To investigate the cause of the elevated LAP expression, we measured the serum concentrations of several cytokines. The granulocyte colony-stimulating factor (G-CSF) level was significantly elevated in MDS patients, and the serum G-CSF concentration clearly correlated with the percentage of LAP-positive cells. Thus, the LAP activity in RA is higher than in normal subjects, and G-CSF is thought to be one of the causes stimulating LAP expression in MDS neutrophils.