SummaryReasons for performing study: Although low birthweight is a risk factor for neonatal illness, the impact of high birthweight on the health of foals and mares, and on the foals' long-term athletic capability, is unknown. Objectives: To investigate whether: 1) foals that are excessively heavy are associated with an increased prevalence of maternal illness in the first month post partum and reproductive dysfunction in the following season; 2) excessively light or heavy foals are at an increased risk of illness in the first month of life; and 3) birthweight is associated with racing success. Methods: Veterinary records from 230 Thoroughbred mares and 409 foalings were reviewed. Data relating to foaling, foal and mare health during the first month post partum and subsequent breeding efficiency were extracted. Foals' racing records were obtained and the association between birthweight and these outcomes examined. Results: Mares with heavy foals had a higher prevalence of minor nonreproductive problems than those with light and average foals. Compared to average foals, placental weight was lower in the light group and higher in the heavy group. Light foals took longer to stand, nurse and pass meconium, whereas gestation period and duration of Stage 2 parturition were longer in heavy foals. Heavy foals were at increased risk of nonseptic musculoskeletal conditions and conformational defects. Birthweight was not associated with racing outcomes. Conclusions: High birthweight was associated with increased prevalence of neonatal musculoskeletal conditions but birthweight did not have a major impact on mares' health and reproductive efficiency, or foals' racing success. Potential relevance: These data add to the understanding of the impact of birthweight on short-and long-term health outcomes in mares and foals.