We propose a hypothesis to explain the temporal and spatial patterns of mass flowerings in dipterocarp tree species on the Malay Peninsula. The literature on these mass flowerings reveals that during 1980-2002 at least 11 flowerings occurred at irregular intervals of 1-6 yr in a lowland rain forest. Five of them were typical mass flowerings-a high density of flowering trees and the characteristic sequential flowering of Shorea species. The 11 flowerings were classified into two flowering times: spring and autumn. There is evidence that low temperature and drought triggered the flowerings. Therefore, the seasonality of mass flowerings is characterized by the annual patterns of rainfall and low temperature. In addition, changes in El Niño-Southern Oscillation (ENSO) may play important roles in determining the supra-annual occurrence of mass flowerings. Flowering surveys on the Malay Peninsula implied that regions with spring or autumn mass flowerings corresponded geographically to those regions that had one cool season (December-February) or two (December-February and June-August), respectively. This finding anticipates the seasonal pattern and geographical distribution of mass flowerings on the Malay Peninsula.