The causative factors of an epidemic of falciparum malaria were investigated in Balcad, Somalia, a town with previously low malaria transmission, where malaria incidence rose more than twenty-fold between 1986 and 1988. The emergence of chloroquine resistance, accelerated by high drug pressure, low herd immunity and favourable meteorological conditions were identified as major causes of the epidemic. Chloroquine resistance of grades RII and RIII was first observed in Balcad in 1987 and rapidly increased to 72% of the Plasmodium falciparum infections in 1988. In the absence of alternative treatment, resistance resulted in the accumulation of a massive infective reservoir and therefore increased malaria transmission, associated with intensive clinical symptomatology. The advent of chloroquine resistance was less violent in the area of Malable, where malaria is stable and communal immunity higher than in Balcad.