"Nepenthes longifolia" is thought to be most closely related to "N. sumatrana"; the stem and lamina of these species are virtually identical in appearance. Nevertheless, "N. longifolia" can be distinguished from "N. sumatrana" on the basis of a number of significant and stable morphological differences. The tendrils on rosette leaves of "N. longifolia" are exceptionally long, reaching 110 cm, while those of "N. sumatrana" are not known to exceed 60 cm. As a result, the lower pitchers of "N. longifolia" are usually around 1/10 the length of the tendril, compared to 1/5 in the case of "N. sumatrana". Although the lower pitchers on immature rosettes are similar in general morphology, the species differ in the shape of lower pitchers on rosettes sprouting from mature plants. Those of "N. sumatrana" are ovoid throughout, with an orbicular lid and the hip immediately beneath the peristome, and are contracted at an angle of 45° to the mouth. Those of "N. longifolia" are ovoid in the lower parts, having the hip around the middle and an ovate lid. In addition, the upper pitchers of "N. longifolia" do not give off a noticeable smell, whereas those of "N. sumatrana" have a sweet, fruity fragrance. In addition, the upper pitchers of "N. longifolia" are only infundibular in the lower parts, compared to the wholly infundibular aerial pitchers of "N. sumatrana". Furthermore, the peristome of "N. longifolia", although distinctly notched, is never raised at the front like in "N. rafflesisna", unlike that of "N. sumatrana", which has a very pronounced raised section. In their description of "N. longifolia", Nerz and Wistuba also used the structure of the inflorescence to distinguish these taxa, but subsequent observations have shown that both species produce one- and two-flowered partial peduncles.