Background and Objective: Infant exposure to environmental tobacco smoke is a major public health problem. The aim of this study was to determine the effects of environmental tobacco smoke exposure on infant weight. Materials and Methods: This was a prospective cohort study. Participants of this study were 51 exposed and 51 non-exposed infants whose weight was measured in three periods of 5-7 days of age and also 2 and 4 months of age. Environmental tobacco smoke exposure of infants and their urinary cotinine were assessed by a questionnaire. The urine samples were collected during each follow-up. For data analysis, independent t test, chi-square and Fisher's Kappa tests were used. Results: Mean urinary cotinine levels over 5-7 days, 2 months and 4 months of age in the exposed group was significantly higher than in the non-exposed group (P<0.001). The mean weight of the exposed group was significantly lower than the non-exposed group in 2 and 4 months of age (P<0.001(. Conclusion: The results of this study show the weigh loss among infants exposed to environmental tobacco smoke, so we suggest further studies with longer follow-up periods and in all aspects of infants’ life.