Social class and ethnicity are important risk factors for small-for-gestational-age and preterm delivery in many countries. This study was performed to assess whether this is also the case in the Netherlands, a country with a high level of social security, relatively small income differences and easy access to medical care for all its inhabitants. Other risk factors that were taken into account were smoking, drinking, occupation, age and height. Information was collected by interview in the first 3 weeks of life of the mothers of 2027 (response 97%) live-born singletons born in the period from April 1988 to October 1989 in the study area. After adjustment for possible confounding factors very low social class, compared with high social class, was significantly associated with reduced birthweight (-4.0%; 95% CI, -7.4% to -0.7%), but not with preterm delivery (OR, 2.09; 95% CI, 0.67-6.48). The adjusted birthweight of Turkish infants (2.7%; 95 CI, -1.1% to 6.5%) and the adjusted birthweight of infants from Suriname or the Antilles (-1.6%; 95% CI, -5.5% to 2.1%) were not significantly different compared with infants of Dutch mothers. After adjustment, the frequency of preterm birth was lower in Turkish infants, but not significantly (OR, 0.22; 95% CI, 0.04-1.10), whereas the frequency of preterm birth in infants from Suriname or the Antilles was significantly higher (OR, 2.51; 95% CI, 1.04-6.08) compared with Dutch infants. Of the other factors the main risk factors were smoking (negatively related with birthweight) and maternal age. Mothers aged 40 years and older had an increased risk (OR, 5.53; 95% CI, 1.72-17.77) of preterm delivery compared with mothers of 20-29 years. After adjustment, higher maternal height was significantly associated with higher birthweight, but also with a decreased risk of preterm delivery. We conclude that in the Netherlands infants of very low social class are at increased risk for low birthweight for gestational age and that a background from Suriname or the Antilles is associated with an increased risk for preterm delivery. Taller mothers not only have heavier infants, but these infants also appear to be less often premature. Older mothers are at increased risk for preterm delivery.