PURPOSE: To study the association between maternal smoking during pregnancy and low birth weight (LBW), small-for-gestational-age birth weight (SGA) and preterm birth, and to quantify the population-attributable fractions for these outcomes in Switzerland. METHODS: Data were gathered for all births in the Canton of Vaud (Switzerland) over a twelvemonth period in 1993-1994. LBW was defined as birth weight <2500 g, SGA as a birth weight <10th percentile for gestational age, and preterm birth as a birth occurring at a gestational age <37 weeks. Maternal smoking before and during pregnancy was recorded. RESULTS : Of a total of 6284 singleton births, 303 (4.8%) were LBW, 731 (11.7%) were SGA, and 364 (5.8%) were preterm. 19.1% of the mothers reported smoking during pregnancy ("smokers"). Mean birth weight, adjusted for maternal age, parity, parents' occupation and neonates' sex and nationality, was lower by 190 g (95% confidence interval: 150-220) in babies of smokers than those of non-smokers. Comparing smokers to non-smokers, the adjusted odds ratios were 2.7 (2.1-3.5) for LBW, 2.1 (1.7-2.5) for SGA and 1.4 (1.1-1.9) for preterm birth. Past smoking was not associated with the outcomes. Maternal smoking during pregnancy accounted for 22% (15-29%) of all LBW babies in the population, 14% (10-18%) of SGA and 7% (1-12%) of preterm births. CONCLUSION: Maternal smoking during pregnancy was closely associated with LBW, SGA and preterm birth. A large proportion of these perinatal outcomes could have been prevented in Switzerland if maternal smoking had been avoided. [Authors]