The study of perinatal mortality requires a sound understanding of the influence of birthweight on perinatal mortality. This paper discusses one aspect of this problem--the pattern of weight-specific mortality. Mortality is very high at the lowest birthweights, falls to a minimum within the range of the most frequent birthweights, but rises again for the heaviest birthweights. Such a curve is best displayed and modelled by plotting the ratio of deaths to survivors on a logarithmic scale. Transformed in this way, perinatal risk may be regarded as the sum of three components--one independent of birthweight, one which decreases linearly with birthweight and one which increases linearly with birthweight. These two lines appear to have slopes of equal magnitude. Each is shown to represent general susceptibility to perinatal problems, rather than the cumulative effect of diseases specific to low birthweight or to high birthweight.