In 1828 Guislain became head of the psychiatric hospitals of Ghent, for which he wrote a new internal regulation together with Petrus Josef Triest.

Joseph Guislain published his Traité sur les phrénopathies in 1833, in which he proposed a new form of psychiatric classification.

This gave rise to the psychiatric doctrine of unitary psychosis which was highly influential in German psychiatry from the mid-nineteenth century.

[1] In his three-volume work Leçons orales sur les phrénopathies of 1852 he further expanded his vision on mental illness.

In 1852, the municipality Council of Ghent accepted his plans for a new psychiatric hospital and in 1857 the Guislain Institute was inaugurated.