BACKGROUND Breast milk contains several immune modulator components. The transfer of numerous cytokines via mother's milk may add to an active stimulation of the infant's immune system. There are many factors in breast milk that could either facilitate or inhibit cytokine activities. Smoking negatively influences the immune system and changes the concentrations of important cytokines. OBJECTIVE The objective of this study was to assess the effect of smoking during pregnancy on the cytokines found in colostrum and mature human milk. METHODS The study population included 25 smoker and 27 non-smoker nursing mothers who gave birth to a term healthy infant via cesarean section. Breast milk was collected from the mothers on the 2(nd)-3(rd) and 21(st)-25(th) days postpartum during visits to examine the newborns. Samples were analyzed for IL-1β, IL-2, IL-4, IL-5, IL-6, IL-8, IL-10, TNF-α and TNF-β cytokines by flow cytometric bead array. RESULTS We first saw that concentrations of IL-1 β, IL-2, IL-4, IL-5, IL-6, IL-8, IL-10, IFN-γ, TNF-α, and TNF-β cytokines, but not IL-12, were measurable both in colostrum and in mature milk, being higher in colostrum. Next we observed that IL-1β and IL-8 levels were significantly lower in colostrum, and IL-6 was found to be significantly lower in the mature milk of smoking mothers. No significant effects of maternal smoking on breast milk concentrations of IL-2, IL-4, IL-5, IL-10, IFN-γ, TNF-α, and TNF-β were observed. CONCLUSIONS These findings indicate that maternal smoking alters the colostrum and mature milk levels of some cytokines. Therefore, it is thought that active smoking during pregnancy decreases the concentration of certain cytokines in breast milk, which might account for the newborn's increased susceptibility to infections.