OBJECTIVE We hypothesized that the fatty acid composition of breast milk can be affected by a smoking habit in the mother. Consequently, this study verified whether maternal smoking modulates, and if so to what extent, the breast milk fatty acid profile. METHODS The study included 20 postpartum women who declared smoking more than five cigarettes daily throughout a period of pregnancy and lactation, and 136 nonsmoking postpartum women. Breast milk samples were collected between the 17th and the 30th day after delivery. The samples were analysed by means of high-resolution gas chromatography for overall content of saturated, monounsaturated and polyunsaturated fatty acids. RESULTS Compared with nonsmokers, smokers were characterized by significantly higher relative breast milk contents of fatty acids and monounsaturated fatty acids. Additionally, smokers' breast milk had higher concentrations of selected saturated fatty acids, monounsaturated fatty acids and polyunsaturated fatty acids. Furthermore, smokers were characterized by significantly lower values of linoleic to arachidonic acid ratio and n-6 to n-3 polyunsaturated fatty acids ratio. CONCLUSIONS Aside from its other harmful consequences, smoking modulates the fatty acid profile of human milk.