In hematological diseases such as myeloproliferative disorders (MPD) or myelodysplastic syndromes (MDS), some abnormalities in the chemiluminescence of neutrophils are observed. There are two groups; one includes chronic myelogenous leukemia (CML), essential thrombocythemia (ET) and MDS, which all have decreased chemiluminescence of neutrophils. The other group includes polycythemia vera (PV) which has increased neutrophil chemiluminescence. We studied the neutrophil function by analyzing the chemiluminescence in 35 patients with hematological diseases. In most of these cases the defects in chemiluminescence in response to N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP) were correlated with those in response to phorbol 12-myristate 13-acetate (PMA). But there were exceptional cases in which the maximal light emission of chemiluminescence (Max CL) in response to FMLP was obviously lower than controls despite the fact that the Max CL in response to PMA was the same as the controls. These facts suggest a heterogenicity of the defect site in these diseases. There was a correlation between the level of chemiluminescence and the neutrophil alkaline phosphatase (NAP) activity in these patients. In vitro culture of CML neutrophils with granulocyte colonystimulating factor (G-CSF) showed a correlation between the increase in the level of chemiluminescence and NAP activity. These results suggest that NAP may take part in the control of neutrophil function. (Internal Medicine 32: 204-209, 1993)