The NAD+–sirtuin 1 (SIRT1) pathway is essential for neutrophil differentiation induced by the granulocyte-colony stimulating factor, and thus may contribute to neutropenia observed after cancer chemotherapies. This study aimed to investigate the relationship between neutropenia and indices for the activity of the NAD-SIRT1 pathway, i.e., nicotinamide phosphoribosyltransferase (NAMPT) activity and SIRT1 expression in patients receiving chemotherapy for lung cancer. Blood samples were collected from sixteen patients at the start of chemotherapy, at the nadir, and before the start of the second chemotherapy course. Neutrophil counts, NAMPT activity and SIRT1 expression level were measured. There were no differences observed either in NAMPT activity or in SIRT1 expression level between the cases of grade 3–4 neutropenia and those of grade 0–2 neutropenia though the SIRT1 expression level tended to decrease at the nadir. Further studies are required to elucidate this relationship between post-chemotherapy neutropenia and SIRT1 expression.