Following a striking increase in the severity of autumnal outbreaks of Plasmodium falciparum during the last decade in the Northwest Frontier Province (NWFP) of Pakistan, the role of climatologic variables was investigated. A multivariate analysis showed that during the transmission season of P. falciparum, the amount of rainfall in September and October, the temperature in November and December, and the humidity in December were all correlated (r2 = 0.82) with two measures of P. falciparum, the falciparum rate (percent of slides examined positive for P. falciparum) since 1981 and the annual P. falciparum proportion (percent of all malaria infections diagnosed as P. falciparum) since 1978. Climatologic records since 1876 show an increase in mean November and December temperatures by 2 degrees C and 1.5 degrees C, respectively, and in October rainfall. Mean humidity in December has also been increasing since 1950. These climatologic changes in the area appear to have made conditions for transmission of P. falciparum more favorable, and may account for the increase in incidence observed in the NWFP in recent years.