Minimizing the interval between parturition and successful rebreeding of thoroughbred broodmares is important in maintaining annual foal production and broodmare profitability. Routine postpartum uterine lavage may help achieve this by aiding uterine clearance and involution in the early postpartum period and increasing pregnancy rates. This retrospective study investigates the effect of postpartum uterine lavage in 32 thoroughbred mares housed on a single farm in Australia in 2014 and 2015. In the control year (2014), postpartum uterine lavage was not performed. In the treatment year (2015), all mares received a uterine lavage 3 days postpartum. In both years, endometrial swabs were obtained for culture and cytology at the first postpartum estrus (foal heat swab) and the second postpartum estrus (second heat swab), prior to the mare being bred by natural cover at the second postpartum estrus. There was no significant difference in bacterial culture rates from foal heat or second heat swabs between the control and treatment years. Assessment of endometrial cytology of the foal heat swab and second heat swab revealed a significantly higher proportion of polymorphonuclear cells (PMNs) in the control year compared with the treatment year. Positive culture and cytology rates were significantly higher at foal heat than at the second postpartum heat in both years. No association of year, cytology, or culture with pregnancy rates was identified. In conclusion, routine postpartum uterine lavage significantly reduces the presence of endometrial PMNs at first and second postpartum estrus but does not reduce endometrial culture rates or increase pregnancy rates.