Objectives: To evaluate the safety and efficacy of four different regimens of didanosine (ddI) +stavudine (d4T) in HIV-infected Thais.Design: Prospective, open-label, randomized study.Methods: Patients were randomized to four regimens of high and low doses of ddI and d4T or to ddI alone. D4T was added to the ddI-alone arm after week 24. The duration of study was 48 weeks.Results: Seventy-eight patients were randomized (mean CD4 cell count, 255 ´ 106/l; mean plasma HIV-1 RNA; 4.3 log10 copies/ml). In the intent-to-treat analysis, 78% of patients in the pooled combination arms and 20% of the patients in the ddI alone arm (to which d4Twas added after 24 weeks) showed plasma HIV-1 RNA < 500 copies/ml at week 24 (P = 0.001), and 59% versus 53% at week 48, respectively. In addition, the proportion of patients with < 50 HIV-1 RNA copies/ml was 13% versus 7% at week 24 (P = 0.5) and 17% versus 20% at week 48 respectively. At week 24, median CD4 cell count increases from baseline were 101 ´ 106/l in the pooled combination versus 76 ´ 106/l in the ddI alone arm (P=0.78). Logistic regression modeling suggested a correlation between receiving high dose ddI and achieving HIV-1 RNA < 500 copies/ml at week 48 (P = 0.07).Conclusions: The d4T/ddI combination was superior to ddI alone in producing HIV-1 viral suppression. At week 48, 60% of patients treated with this combination reached HIV-1 RNA levels< 500 copies/ml. Receiving high dose ddI but not d4T may correlate with a better viral suppression.