Cigarette smoking during pregnancy is an important, avoidable factor associated with low birth weight. Maternal age is also associated with variations in birth weight. Using birth certificate data from all 347,650 singleton births for which maternal age and birth weight were recorded during 1984-1988 in Washington State, this study investigated birth weight and smoking during pregnancy (yes/no) for mothers of different ages. In multiple linear regressions adjusted for race, marital status, parity, adequacy of prenatal care, and urban/rural residence, the decrement in mean birth weight associated with smoking grew steadily from 117 g for the youngest mothers (age less than 16 years) to 376 g for the oldest (age 40 years or more). Similarly, the adjusted relative risk of having a low weight birth (less than 2,500 g) for smokers compared with nonsmokers was lowest for mothers aged 16-17 years, at 1.43 (95% confidence interval 1.22-1.68), and increased steadily to 2.63 (95% confidence interval 1.77-3.90) for mothers aged 40 or more. This result suggests that the effect of exposure to cigarette smoking during pregnancy is modified by advancing maternal age. Further research using data that more precisely measure the exposure (cigarettes per day, years smoked) could help further clarify this