It is important to point out to professionals involved in Public Health, especially in malaria control, that the high frequency of CYP2D6*4 in Goianésia do Pará, PA, (about 1/3 of the population) results in a chance of relapse during treatment with the current therapeutic protocol since carrying this polymorphism results in a poor metabolizer phenotype. The gene discussed is CYP2D6; the disease is malaria.