The recurrent great malaria epidemics which occurred in the Punjab province of former British India and Ceylon before the introduction of residual insecticides have been related to excessive and failing monsoon rains respectively. In the arid Punjab, rainfall facilitated breeding and increased the lifespan of the mosquito vector and, in the wet part of Ceylon, failing monsoon rains caused rivers to pool, creating more favourable breeding conditions. The periodic fluctuations in monsoon rainfall and epidemic malaria are here explained in relation to the El Niño Southern Oscillation. In the Punjab, epidemic malaria between 1868 and 1943 correlates significantly (r = 0.34, P < 0.005) with the sea surface temperature anomalies in the Eastern Equatorial Pacific, a parameter of the oscillation, and epidemics were significantly more prevalent in a year with a wet monsoon following a dry El Niño year than in other years. In Ceylon, epidemics were significantly more prevalent during El Niño years, when the same south-west monsoon tends to fail. With the reduced reliance on residual insecticides and the recurrence of epidemic malaria on the Indian subcontinent, advances made in predicting El Niño events may be used to forecast high and low risk years for future malaria epidemics.