The relationship between parental social position and birth weight is being studied. A total population of 102,638 single born, first-born infants was included in the study. Census information from 1980 was linked to the Medical Birth Registry for the years 1978-1982. Children of parents with a high education (more than 15 years) had the highest birth weight. The association was distinct and linear between low birth weight and education. The same tendency appeared for paternal socio-economic status, but differences were comparatively small. When examining income the pattern was different. The highest maternal income group had the highest proportion of low birth weight offspring. When examining parental education jointly, it was found that the mothers' education had the greatest impact on birth weight. By adjusting for female smoking, using survey data, the association between maternal education and birth weight was weakened. However, assuming that birth weight is decreased by 200 g from smoking, the effect was still significant.