The aim of this study was to determine the independent effects of climatic factors: rainfall, temperature and relative humidity, on the occurrence of dengue hemorrhagic fever (DHF) in Thailand after adjustment for cyclical pattern. Data regarding monthly DHF reported cases by province, monthly rainfall, rain-days, average daily maximum temperature, average daily minimum temperature and average relative humidity, and mid-year province population from 1978 to 1997 (240 months) in 73 provinces were collected from various governmental departments. The general equation: Incidence of DHF = constant + trends + cyclic effects + climatic factors + noise was used as the statistical model. Out of 73 provinces examined, an increase in temperature was associated with a rise in the incidence of DHF in 9 provinces, whereas increased rainfall was associated with a decreased incidence of DHF in 7 provinces. Analysis by region shows that DHF incidence was negatively associated with extra rainfall in the southern region, but was positively associated with elevated temperatures in the central and northern regions. Variability in incidence was explained mostly (14.7% to 75.3%) by trend and cyclic change and much less (0.2% to 3.6%) by independent climatic factors.