Passive smoking was long overlooked by those in the medical and legal professions as being harmful to one's health, but in recent years the negative effect of passive smoking has come to the fore in the media and laws have been changed so that less people are obliged to unwillingly suffer from passive smoking, particularly in the workplace and in indoor settings. To study the effects of environmental tobacco smoking exposure during the breast-feeding period on maternal milk lipids. This cohort study was conducted on 45 mothers environmental tobacco smoking exposure and 40 non-exposed post-partum mothers referred to the Shahid Ayat health center, Tehran, Iran. Socioeconomic conditions and the demographic characteristics of exposed and non-exposed groups were recorded. Milk samples were collected twice--at baseline (5-7 days after delivery) and four months after delivery. The samples were reserved at -20°C until assay. Milk lipids including cholesterol, triglyceride (TG), high density lipoprotein (HDL) and low density lipoprotein (LDL) were evaluated. Dietary intake assessment was performed by means of the 24-hour dietary recall questionnaire both times. Maternal occupation status and education levels were significantly different between the two groups. Lipids profiles of milk were significantly higher 5-7 days after delivery in the non-exposed group and four months after delivery. Dietary intake was not significantly different between the two groups. Maternal environmental tobacco smoking exposure affects milk lipids which are essential for infant growth.