Primaquine phosphate has been used to prevent relapse as a radical cure after the acute-phase treatment of vivax and ovale malaria however. Many vivax malaria relapses have been reported following a standard dose of primaquine (15 mg/day for 14 days). A higher dose of primaquine (30 mg/day for 14 days) decreases the relapse rate, and the concomitant risk of gastrointestinal side effects tends to disappear when the drug is administered with food. G6PD deficiency is rare in the Japanese population. Although the relapsed phenomenon is reported globally, the higher dose of primaquine is currently recommended in Japan only for those returning from Southeast Asia or Papua New Guinea. Cases of 18 Japanese, including 13 vivax malaria and 5 ovale malaria, prescribed primaquine at a referral center in Japan, were analyzed retrospectively from 2007-2011. Data on diagnosis, treatment, and outcome were extracted from medical records. Of the 18, 10 with vivax malaria were administered the higher dose of primaquine. We found that only one suffered relapse-a vivax malarial case returning from Brazil and treated with the standard dose of primaquine. No ovale malarial case suffered relapse. None, including the 10 prescribed the higher primaquine dose, experienced any adverse side effects. Based on our findings, we recommend a higher dose of primaquine be used to prevent relapse when treating Japanese suffering from vivax malaria.