Objective: To estimate the cost-effectiveness of alternative interventions to reduce the risk of mother-to-child transmission of HIV. Design: A model capturing the sequential nature of mother-to-child transmission in utero, at delivery and postnatally was used to determine how the effects of bottle-feeding, elective Cesarean section (CS) and zidovudine (ZDV) would combine to prevent mother-to-child HIV transmission. Parameter estimates were derived from the literature, UK health service costs applied, and incremental cost effectiveness ratios (ICER) estimated for alternative risk reduction strategies. Results can be transposed to other cost assumptions or currencies. Results: In a woman who breast-feeds her baby, has a vaginal or emergency CS delivery and takes no ZDV, the estimated transmission risk is 31.6% (range, 23.7-38.1%), at a cost of UK pound 400 per woman; this falls to a risk of 3.7% (range, 1.7-6.9%) when bottle-feeding, ZDV therapy and elective CS are all implemented at a cost of UK pound 1968 per woman. From a public health perspective the ICER of ZDV and elective CS each depend on the acceptance rates of the other. In women counselled against breast-feeding, ZDV with 100% acceptance of elective CS has an ICER of UK pound 11 342 [95% confidence interval (CI), UK pound 7084-21 515]. However, the ICER of CS ranges from UK pound 9248 (95% CI, pound 5072-46 913) at zero ZDV acceptance to UK pound 27 895 (95% CI, pound 10 018-154 462) at 100% ZDV acceptance. Conclusions: Considering the estimated cost of caring for an infected child, ZDV appears to be cost-effective under any of the circumstances examined. However, elective CS may not be cost-effective in populations where the uptake of ZDV is high, and a more precise estimate of its efficacy is required. (C) 1998 Lippincott-Raven Publishers.