Clarifying variations in soil moisture is an important part of hydrological and ecological studies in tropical rainforests. Volumetric soil water content (VSWC) was measured at depths of 10, 20 and 30 cm at Pasoh Forest Reserve (PFR) in Peninsular Malaysia for 12 years. VSWC ranged from 0.280-0.442 m(3) m(-3) (mean +/- standard error; 0.373 +/- 0.001 m(3) m(-3)) and was affected not only by the south-west and north-east monsoons but also by El Nino and La Nina events. Mean VSWC was high during La Nina events and low during El Nino events. In a normal El Nino/Southern Oscillation (ENSO) phase, VSWC showed both low and high conditions. An antecedent precipitation index (API) was calculated using 20 years of rainfall data. API(90) ranged from 1.2-257.3 mm (25.4 +/- 0.2 mm). The variations in API(90) followed a similar pattern to variations in the VSWC. API(90) can be used to estimate soil moisture conditions as a drought indicator. An API(90) of less than 10 mm lasted more than 14 successive days during the normal ENSO phases when five mass flowerings occurred at PFR from 1995-2014.