Two patients, a 25-year-old woman and a 62-year-old man, with Plasmodium vivax infection, suffered repeated attacks of malaria despite standard treatment with chloroquine and subsequently primaquine. The relapses were due to a combination of decreased primaquine sensitivity and incorrect prescription of primaquine. Primaquine is administered as diphosphate (salt), but the required amount has to be prescribed as base (26.3 mg diphosphate corresponds to 15 mg base).