Case series data suggest that high birth weight and neonatal macrosomia in offspring are associated with an increased risk of colorectal cancer in parents. We therefore conducted a prospective analysis investigating the association among self-reported birth weight, neonatal macrosomia, and incident colorectal cancer in a population-based study of men and women. Participants were drawn from a cohort of men and women ages between 45 and 79 years: the European Prospective Investigation of Cancer in Norfolk study. A total of 4532 men and 7325 women who reported their birth weight were followed up between 1993 and 1999. The relation between birth weight and incident colorectal cancer was assessed using Cox's proportional hazards model. All Ps are two-sided. The association between self-reported birth weight and risk of incident colorectal cancer was nonlinear. Relative to individuals born weighing 2500-3249 g, the adjusted hazard ratio for people born with neonatal macrosomia was 2.57 (95% confidence interval = 1.15-5.74). There was also some evidence that low birth weight babies were at increased risk of colorectal cancer relative to the referent category. These risks were essentially unaltered after adjustment for potential confounders. There is a J-shape relation between self-reported birth weight and subsequent risk of colorectal cancer. Babies born with macrosomia appear to have the greatest risk.