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Objectives To evaluate the impact of the modified ACTG 076 zidovudine regimen on the risk for vertical HIV transmission. Design Observational retrospective evaluation of a prospective cohort. Setting Thai Red Cross zidovudine donation program to reduce vertical HIV transmission. Patients HIV-infected Thai women and their offspring. Intervention The modified regimen consisted of 500 mg zidovudine daily during pregnancy and 300 mg zidovudine every 3 h during labor, taken orally, and 2 mg/kg zidovudine syrup four times daily for 6 weeks to infants. Main outcome measures Only infants with at least 1 HIV DNA polymerase chain reaction (PCR) result at age > or = 4 weeks were included. HIV infection was defined by having at least one positive PCR at age > or = 4 weeks. The transmission rate was calculated. Characteristics of women who did and did not transmit HIV to infants were compared. Results A total of 2891 women and their infants participated in the program and 726 infants of 719 women were included in the analysis. Forty-three infants were infected. The overall transmission rate was 6.0% (95% confidence interval, 4.4-8.0). There were no differences in maternal characteristics between transmitters and non-transmitters. The transmission rate in women who started zidovudine before 30 weeks' gestation was not significantly different from that in women who started zidovudine at or after 30 weeks' gestation: 5.7 versus 3.3%, respectively. Conclusions This modified zidovudine regimen is effective in reducing vertical transmission in a country with predominant subtype E infection. A donation program for preventing vertical HIV transmission can be implemented in developing countries, as in Thailand.