OBJECTIVE To prospectively examine dose-response and timing effects of prenatal (PTE) and postnatal tobacco exposure on obesity risk assessed by conditional weight-for-length gain (CWFLG), by 2 years of age. METHODS CWFLG over the first 2 years of life was examined for 117 PTE and 57 nonexposed children. Repeated assessments of PTE were conducted beginning in the first trimester of pregnancy, using multiple methods. RESULTS PTE or postnatal exposure status was not predictive of CWFLG. However, there was a dose-response association and an association with fetal exposure ascertained by infant meconium positive for nicotine and metabolites. CONCLUSIONS PTE is related to restricted growth at birth, yet associated with accelerated CWFLG by 2 years of age, a measure that controls for birthweight differences. Results highlight the importance of examining dose-response and timing of exposure associations, along with the importance of obesity risk-reduction interventions within the first 2 years of life among PTE children.