Chloroquine-resistant Plasmodium vivax has not yet occurred in Vietnam. The efficacy of artemisinin for P. vivax was not established. We conducted a double-blind randomized study involving 240 inpatients with P. vivax malaria who received artemisinin (40 mg/kg over 3 days) plus placebo chloroquine (Art) or chloroquine (25 mg/kg over 3 days) plus placebo artemisinin (Chl). Patients were followed up with weekly blood smears for 28 days. In each group 113 cases were analysed. All patients recovered rapidly. The median (range) parasite clearance time of regimen Art was 24 h (8-72) and of Chl 24 h (8-64; P = 0.3). Parasites reappeared in two cases in each group on day 14, in eight cases in each group (7%) on day 16 and in 25 (23%) and 18 (16%) cases, respectively, at the end of 4-week follow-up (P = 0.3). The population parasite clearance curve followed a mono-exponential decline. The parasite reduction ratio per 48 h reproduction cycle was 2.3 x 104 for both regimens. We conclude that artemisinin and chloroquine are equally effective in the treatment of P. vivax infections in Vietnam. Reappearance of parasites before day 16 (7%) suggests the emergence of chloroquine resistance. Three days of artemisinin monotherapy does not prevent recrudescence.