Fifty patients with Plasmodium vivax infection were treated with the standard regimen of chloroquine phosphate (1500 mg over 3 d) followed by primaquine (45 mg immediately and then weekly for 8 weeks); 43 patients had sensitive infections but recrudescences of parasitaemia occurred between days 3 and 14 with RI, RII and RIII patterns in one, 3 and 3 patients, respectively. All the chloroquine-resistant cases were again treated with chloroquine (1500 mg) and no further recrudescence or relapse was detected on days 21 and 28. This study indicates that chloroquine is losing its efficacy against P. vivax in Myanmar.