We developed a stem diameter D-height H allometric model that included variability in the D-H relationship along a topographic gradient. The study site was located along a belt transect (2500 × 20 m) established in a primary tropical forest near Manaus, Brazil. The transect included typical topography of the region, characterized by plateaus and valleys called “baixios”. The D-H allometric model (n = 1307) indicated that potential tree height increased significantly, from 28 m at the lowest baixio plot to 35 m at the highest plateau plot. Consequently, by combining the D-H allometric model and an allometric equation with the variable D²H, biomass was estimated for trees (D > 10 cm) in each sub-plot (20 × 20 m). Ignoring variability in the D-H relationship introduced wide-ranging error to biomass estimation; error values ranged from -5% at a baixio plot to +6% at a plateau plot. Average biomass was 317 ± 28 (SE) Mg ha^-1, and tree density and biomass fell significantly with decreasing relative elevation.