Patients harbouring Loa loa and/or Mansonella perstans infections, or presenting typical filarial symptomatology, were treated with mebendazole, a drug shown to be efficient in other parasitoses. Of the patients who took the drug during 21 days and provided regular blood samples, 84% showed a significant decrease in their L. loa microfilaraemia and 100% in their M. perstans microfilaraemia, with a concomitant improvement in the clinical manifestations and no side reaction to drug administration. Additionally, the long-lasting effect of the drug (up to day 200 in some cases) may suggest a positive action on the adult worm. Mebendazole may consequently be regarded as a safer alternative to diethylcarbamazine for the treatment of these two parasitoses.