Active management of the third stage of labour (AMTSL) using oxytocin substantially reduces postpartum haemorrhage (PPH), a leading cause of maternal mortality. An economic analysis of the use of AMTSL was conducted as part of an intervention study in Thanh Hoa Province, Vietnam. A spreadsheet was used to calculate various scenarios and estimate the costs and outcomes of the routine use of AMTSL with oxytocin in Uniject compared with oxytocin in ampoules, and AMTSL compared with no AMTSL. We estimated the health outcomes from probabilities that were generated from the effectiveness portion of the AMTSL intervention project. The study also estimates the costs of treating PPH and the net incremental costs of AMTSL (costs and savings); examines the impact of different scenarios of PPH rate and Uniject cost; and estimates the potential cost per PPH case and PPH death averted. The additional net cost per woman of providing AMTSL with ampoules was just US dollar 0.20 in the base case; using Uniject devices added only US dollar 0.08 more per woman to the ampoule cost. Varying the rate of PPH had the biggest effect; if the underlying PPH rate were 8%, the incremental cost of AMTSL drops to just US dollar 0.07 per woman with ampoules and the cost to avert a case of PPH is US dollar 2.10 with ampoules and US dollar 4.52 with Uniject. The low net incremental cost of AMTSL suggests that the introduction of AMTSL in primary-level facilities in Vietnam can reduce the incidence of PPH and benefit women's health without adding much to national health care costs.