At a time when Fansimef, the fixed combination of mefloquine, sulfadoxine and pyrimethamine was considered for prophylaxis of falciparum malaria, a randomized double-blind study comparing the efficacy and tolerability of Fansimef with that of Lariam (mefloquine), Fansidar, chloroquine and placebo in malaria prophylaxis was performed in Thailand from July 1987 to January 1988. The study population of 602 adult males was recruited in Pak Tongchai District, some 360 km North-East of Bangkok, where multiresistant P. falciparum is endemic. All active treatments and placebo were given once weekly for 24 weeks with doses as follows: Fansimef: 125 mg mefloquine + 250 mg sulfadoxine + 12.5 mg pyrimethamine (1 half-strength tablet); Lariam: 125 mg mefloquine (1 half-strength tablet); Fansidar: 500 mg sulfadoxine + 25 mg pyrimethamine; chloroquine; 300 mg. A loading dose of 2 half-strength tablets was given in the Fansimef group in weeks 1 and 2 and in the Lariam group in weeks 1 to 4. The incidence of acute episodes of P. falciparum per 100 person months of prophylaxis was 0.17 each in the Fansimef and the Lariam groups, 1.18 in the Fansidar group, 0.69 in the chloroquine group and 0.64 in the placebo group (differences statistically not significant). Clinically adverse events were reported by 170 subjects (Fansimef 28, Lariam 29, Fansidar 41, choroquine 43, placebo 29; differences statistically not significant). The most frequent adverse events in all groups were headache, sleepiness, dizziness and weakness.(ABSTRACT TRUNCATED AT 250 WORDS)