Primaquine (8-aminoquinoline), the only effective drug to prevent relapses of the persistent liver forms of Plasmodium vivax and Plasmodium ovale, can induce hemolytic anemia in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency. The severity varies considerably among affected individuals. Three hundred and sixty-four Plasmodium vivax cases (342 G6PD-normal and 22 G6PD-deficient) were given a 3-day course of chloroquine (total dose 1,500 mg) followed by primaquine 15 mg a day for 14 days and completed a 28-day follow-up. All G6PD-deficient patients were male; there were no relapses or serious adverse events during the study. Although a significant decrease in hematocrit levels and an increase in the percent reduction of hematocrit levels were observed on day 7 (34.9+/-5.0 vs 26.7+/-5.4; (-1.2)+/-14.4 vs (-24.5) +/-13.9 respectively) and on day 14 (35.7+/-4.3 vs 30.9+/-3.1; 1.6+/-17.8 vs (-11.0) +/-19.3 respectively) blood transfusion was not required. Daily doses of 15 mg of primaquine for 14 days following a full course of chloroquine when prescribed to Thai G6PD deficient patients where Mahidol variant is predominant, are relatively safe.