As part of a study on causes of variation in birth weight, questionnaire data on parental measures were related to offspring birth weights recorded in the Medical Birth Registry of Norway. A genetic analysis of parent-offspring covariances in birth weight indicated that about 60% of the variance in birth weight could be explained by effects of fetal genes, while no effects of maternal genes were detectable. Multiple regression analysis showed that height and weight of both parents and maternal smoking status were associated with variation in birth weight. Socioeconomic status, educational attainment and paternal smoking habit had no independent effects. The adult, parental variables could only explain 10% of the variation in mean offspring birth weight.