We contrasted the structure and biomass of four lowland Neotropical forests (La Selva Costa Rica; Barro Colorado Island Panama; Cocha Cashu Peru; and KM41 Brazil) to determine if commonalities exist within and among forests differing in latitude rainfall seasonality and soil fertility. We examined the effect of soil fertility specifically by measuring the density and basal area of trees lianas and palms on two soil types differing in fertility at each site. We used allometric relationships to estimate the contribution of the various life-forms to total aboveground biomass (AGB) and compared two relationships for trees 30 cm diameter or greater. Estimated liana density and AGB were similar among sites but the density and AGB of trees and palms estimated using diameter alone differed significantly. Basal area and AGB of trees 10 cm diameter at breast height (DBH) or greater differed among forests and averaged 30.2 m2/ha and 250 Mg/ha respectively. Cocha Cashu and KM41 had higher tree basal area and AGB than La Selva or Barro Colorado Island. Across forests lianas and small trees (1–10 cm DBH) each contributed between 4 and 5 percent of the total AGB and small palms contributed ca 1 percent. Many forest inventories ignore lianas as well as trees and palms less than 10 cm DBH and therefore underestimate AGB by ca 10 percent. Soil type had little influence on the forest structure within sites except at Cocha Cashu where total AGB was much higher and liana density much lower on the more fertile old floodplain Entisols than the terra firme Oxisols. Although total stem density basal area and some biomass components differed significantly among forests they seemed less variable than other quantitative measures (e.g. species richness).