De Isla died in Bologna in extreme poverty in 1781. His last book, Cartas de Juan de la Encina (Letters from Juan de la Encina), published posthumously (1784), constituted a sharp condemnation of the practices of the Spanish physicians of his time. In 1803 the priest, José Ignacio de Salas printed his biography in Madrid and in the Ibarra workshop, under the title Compendio histórico de la vida, carácter moral y literario del célebre padre Isla con la noticia analítica de todos sus escritos (Compendium history of the life, moral and literary character of the famous father Isla with analytical notes on all his writings), paid for by his devoted sister, who was also the editor of many other works by her brother.
