Two zidovudine/lamivudine regimens for the prevention of HIV perinatal transmission were studied for the selection of resistance mutations. The M184V mutation was detected one week after delivery in six out of fifty women (12%) who received the regimen prepartum, intrapartum and postpartum, and was no longer present 3 months later. No nucleoside reverse transcriptase inhibitor resistance-associated mutations were detected in 50 women who received zidovudine/lamivudine intrapartum and postpartum only. No association with the risk of perinatal transmission was found.