As malaria caused by Plasmodium falciparum has become resistant to chloroquine alternative drug regimens need to be developed. The prophylactic efficacy against malaria and the side effects of chloroquine phosphate 500 mg weekly with proguanil hydrochloride 200 mg daily was compared with the efficacy of chloroquine 500 mg weekly with sulfadoxine 500 mg-pyrimethamine 25 mg weekly in a randomised study of Scandinavian travellers to Kenya and Tanzania during 1984-5. A total of 767 subjects (416 male and 351 female; 384 taking chloroquine phosphate with proguanil hydrochloride and 383 taking chloroquine with sulfadoxine-pyrimethamine) completed a diary on the breakthrough of malaria and the side effects of treatment while taking the drugs. They were also asked to make thick blood films when symptoms like those of malaria occurred, which were sent to and analysed in Denmark. Four subjects taking chloroquine with proguanil hydrochloride and three taking chloroquine with sulfadoxine-pyrimethamine developed falciparum malaria, which was verified microscopically. Side effects were reported by 36 subjects taking chloroquine phosphate with proguanil hydrochloride and 55 taking the other regimen (p = 0.043). The side effects of both regimens were generally mild, but the combination of chloroquine phosphate with proguanil hydrochloride is recommended because it results in fewer side effects. As breakthroughs of malaria occurred at the earliest after seven weeks self treatment should not be recommended for travellers staying only a short time. Thick blood films are useful for diagnosis of suspected cases of malaria, can be prepared by non-specialists in Africa, and can be analysed successfully after long delays.