The novel ends with a long, poetic sentence lamenting the impossibility of ever understanding the Lisbon girls:
It didn't matter in the end how old they had been, or that they were girls, but only that we had loved them, and that they hadn't heard us calling, still do not hear us, up here in the tree house, with our thinning hair and soft bellies, calling them out of those rooms where they went to be alone for all time, alone in suicide, which is deeper than death, and where we will never find the pieces to put them back together. (5.42)
The ending finally brings us to the present, where we find that the narrators are now middle-aged men rather than teenagers. It's a final abandonment of the novel's project, which is to piece together the explanation for the suicides. There just isn't one.