To clarify consistency in the size of carbon pool of a lowland tropical rainforest, we calculated changes in above-ground biomass in the Pasoh Forest Reserve, Peninsular Malaysia. We estimated the total above-ground biomass of a mature stand using tree census data obtained in a 6-ha plot every 2 years from 1994 to 1998. The total above-ground biomass decreased consistently from 1994 (431 Mg ha(-1)) to 1998 (403 Mg ha(-1)) (1 Mg = 10(3) kg). These are much lower than that in 1973 for a 0.2 ha portion of the same area, suggesting that the the total above-ground biomass reduction might have been consistent in recent decades. This trend contrasted with a major trend for neotropical forests. During 1994-1998, the forest gained 23.0 and 0.88 Mg ha(-1) of the total above-ground biomass by tree growth and recruitment, respectively, and lost 51.9 Mg ha(-1) by mortality. Overall, the biomass decreased by 28.4 Mg ha(-1) (i.e. 7.10 Mg ha(-1).year(-1)), which is almost equivalent to losing a 76-cm-diameter living tree per hectare per year. Analysis of positive and negative components of biomass change revealed that deaths of large trees dominated the total above-ground biomass decrease. The forest biomass also varied spatially, with the total above-ground biomass density ranging 212-655 Mg ha(-1) on a 0.2-ha basis (n= 30 subplots, 1998) and 365-440 Mg ha(-1) on a 1 ha basis. A large decrease of the total above-ground biomass density (> 50 Mg per ha per 2 years) in several 0.2-ha subplots contributed to the overall decrease in the 6-ha total above-ground biomass. In the present study, we discuss the association between forest dynamics and biomass fluctuation, and the implication for carbon cycling in mature forests with emphasis on forest monitoring and assessments of soil and decomposition systems.