I have tried to show, using a contemporary international data set, the overall consistency in shape of curves of national birthweight distributions which reflect the biological and social characteristics of the population from which they are derived, and the effects of changes in these characteristics. For several countries, including the United States and England and Wales, the trends in recent years have been such as to shift the main distribution upwards, so that the median weight has increased. Also shown has been the close and specific relationship within each population group between infant mortality and birthweight, with sharp falls of mortality with increasing birthweight. It has been shown elsewhere that similar patterns are seen with short- and long-term morbidity, thus underlining the importance to be attached to increasing birthweight particularly in underprivileged groups. In the short term this can be done by reducing the frequency of parental smoking, where this is a problem, and in the longer term by improving maternal health and nutrition. The shift towards higher birthweights if it persists, should make an important contribution towards the improvement of the public health of the next generation.