Thus far little is known about the dose-response relationship between birth weight and the amount of maternal smoking during pregnancy. The purpose of this report is to describe the effects of smoking intensity, duration, and timing on birth weight with the use of three measures of exposure: self-reported daily consumption, self-reported cumulative consumption, and salivary thiocyanate. Data were obtained on 867 single live-born infants and their mothers who participated in a randomized anti-smoking intervention trial. Smoking was measured for the women at about 15 weeks gestation and again during the eighth month. Although all indicators of dose, as derived from early or late pregnancy smoking measures, were significantly associated with birth weight, whether or not the mother had quit smoking by the time of the 8th month follow-up was almost as predictive as any dose variable. For women who quit smoking before 30 weeks gestation, neither the duration nor the amount of smoking earlier in pregnancy was an important determinant of birth weight.