Stem development in palms usually involves the building up of a trunk of maximum diameter before emerging from the ground. Unlike most arboreal palms, the oligarchic palm Iriartea deltoidea has been shown to grow in both height and diameter. Nevertheless, the drivers of this unusual growth strategy are poorly known. We examined the growth rates of 554 stems found in 10 ha of the 50-ha Yasun ı Forest Dynamics Plot. We explored whether palm growth responds to: (1) the size of individual palms; (2) light availability; or (3) the number of conspecifics and heterospecifics in the surrounding neighbourhood. Individual growth rates were positively related to palm height in the three life stages considered here (seedlings, saplings and adults). The mean annual diameter increment was higher (3.2 AE 4.6 mm year À1 ) in individuals with a diameter at breast height (DBH) of < 20 cm, and mean vertical elongation of juveniles (0.2 AE 0.2 m year À1 ) was initiated early in development until they became pre-adults. Growth in I. deltoidea was minimally affected by the presence of neighbours (basal area of conspecifics and heterospecifics), suggesting that this species can succeed at high densities with low levels of interspecific competition. Under the forest canopy, growth rates were not significantly different in palms with greater light exposure. Our findings contribute to an understanding of the factors that make I. deltoidea a dominant species in western Amazonia.