Standard treatment of P. ovale and P. vivax malaria is 1500 mg of chloroquine base given over three days followed by 15 mg of primaquine base daily for 14 days. At the Department of Infectious Diseases, Rigshospitalet, Copenhagen an increasing number of cases of relative by primaquine-resistant malaria have been observed. The side effects of primaquine are mainly gastrointestinal. Primaquine may also cause serious toxic side effects, including methaemoglobin formation and haemolytic anaemia, especially in individuals with erythrocyte glucose-6-phosphate dehydrogenase (G-6-PD) deficiency. We present six patients who developed relapses of P. vivax after taking standard doses of primaquine. When increasing primaquine dosage, patients must be informed about the possibility of more severe side effects.