PURPOSE: Studies of smoking during pregnancy and preterm birth among Hispanic women are sparse. Our goal was to evaluate the effect of cigarette smoking during pre-pregnancy, early pregnancy, and mid pregnancy on preterm birth among Hispanic women, the fastest growing ethnic group in the United States. METHODS: We evaluated data from a prospective cohort study of 1,041 Hispanic (predominantly Puerto Rican) women recruited between 2000 and 2004 in Springfield, Massachusetts. At recruitment (mean = 15 weeks), women reported their smoking since pregnancy awareness (early pregnancy) and in the year prior to pregnancy (pre-pregnancy). Mid pregnancy smoking was collected at a second interview (mean = 28 weeks). RESULTS: Smoking in pre-pregnancy was not associated with preterm birth. After adjustment for age, parity, education, and illicit drug use, women who smoked in early pregnancy had 1.6 times the risk of preterm birth (95% confidence interval [CI], 1.0-2.7) compared with nonsmokers. Women who smoked in mid pregnancy had 2.1 times the risk of preterm birth (95% CI, 1.0-4.2) compared with nonsmokers with a trend of increased risk of preterm birth with increasing levels of smoking (p trend, 0.03). CONCLUSIONS: Smoking in early or mid pregnancy increased the risk of spontaneous preterm birth in a Hispanic population.