A study on efficacy and tolerance of Mefloquine was carried out in three large centres in Kenya, the Kenyatta National Hospital in Nairobi, Coast General Hospital in Mombasa and New Nyanza Hospital in Kisumu. A total of 257 patients with uncomplicated acute falciparum malaria were treated with 1000 mg of mefloquine in two split doses 8 hours apart. 253 patients completed the study. All had negative blood clearance time was 18 hours and the mean parasite clearance time was 48 hours. 81 patients from Nairobi were followed up until day 28 and all were cured. The patients in Kisumu. and Mombasa were not followed up until day 28 as these are highly endemic areas and it would have been impossible to rule out reinfection. The drug was well tolerated, the main side effects being dizziness which was noted in 74 patients.