He is known for identifying consonantal reflexes in Hittite that were previously only hypothesized by Ferdinand de Saussure, thereby offering first direct evidence for the laryngeal theory.

This gave him an opportunity to qualify as a university professor of Indo-European linguistics soon after his return to Poland.

His son, also named Jerzy Kuryłowicz (1925–2002), obtained his PhD from the Warsaw University of Technology.

In the latter, he discussed the inflectional categories of Indo-European languages and later, on the basis of these studies, formulated the so-called Case Theory.

[3] The laws consist of six predictive statements about the direction of analogical changes: In this theory he proposes the division into grammatical and concrete cases.

The category of case covers two basic case groups: For example, the Polish verb kierować (to drive) governs the direct object in the instrumental case, as in the expression kierować samochodem (to drive a car) (Fisiak 1975: 60) While studying the phonology of Indo-European languages, Kuryłowicz pointed to the existence of the Hittite consonant ḫ in his 1927 paper "ə indo-européen et ḫ hittite".

Analyzing the problem of hierarchy he introduced the concept of foundation, which is the relation between two forms or functions in a language.