Pulmonary alveolar proteinosis (PAP) is characterized by filling of the alveoli with a periodic acid-Schiff-positive proteinaceous material. Although the pathogenesis of primary or idiopathic PAP remains unknown, it has been proposed that a deficiency or loss of responsiveness of the monocyte/macrophage lineage to granulocyte-macrophage colony stimulating factor (GM-CSF) is involved in PAP. Secondary PAP is associated with haematological malignancies, especially in myeloid disorders. Herein, we report on an adult with PAP associated with myelodysplastic syndrome (MDS). The CD16+ CD14dim monocytes comprise 5-10% of circulating monocytes in healthy volunteers. Flow cytometric analysis of the patient in the present study revealed increased CD16+ CD14dim monocytes in the peripheral blood. It has been demonstrated that the expression of CD16 and CD14 is regulated by macrophage colony stimulating factor (M-CSF) and GM-CSF. Hence, serum cytokines were analysed in our patient and the concentration of serum GM-CSF was found to be less than the lower limit of the assay. In addition, serum M-CSF and granulocyte colony stimulating factor levels were only slightly increased above the normal range. These results suggest that the increase in the CD16+ CD14dim subpopulation in the circulation of our patient indicates another pathogenetic mechanism for secondary PAP, such as hyperresponsiveness of the monocyte/macrophage lineage to these cytokines.