Changes in monocyte functions have been described in several human malignancies. The monocyte/macrophage system is known to play a crucial role in the rejection of tumor cells and phagocytosis represents an important defense mechanism used by these cells. This paper reports the adherence power and phagocyting ability (latex beads) of circulating monocytes in 20 patients with unresectable non-small cell lung cancer (NSCLC), stage IIIB or stage IV, before and after multiagent chemotherapy (carboplatin + etoposide + ifosfamide or cisplatinum + etoposide). We demonstrated that both monocyte adherence and phagocytosis were not affected in lung cancer patients before chemotherapy in comparison with healthy controls. After chemotherapy, a statistically significant decrease in monocyte count on day 4 (p < 0.05) and in their phagocyting ability on day 4 and 15 (p < 0.001 and p < 0.05 respectively) was showed. In addition, a statistically reduced monocyte adherence was found on day 4 (p < 0.05). The described impairment was prolonged but reversible. These changes in monocyte functions after chemotherapy could be due to a direct effect of the chemotherapy on these cells or to functionally immature cells circulating after myelodepression. The in vitro assessment of monocyte functions may be useful to better clarify mechanisms by which anti-neoplastic agents may act on immune functions and prevent adverse side effects.