The emergence of personalized party organizations has called for a redefinition of the concept of party institutionalization (PI). This article brings different strands of literature together and proposes a new multidimensional index of PI. PI is conceived of as the interplay of objective and internal party features with the external environment. The viability of the index is tested through its application to a prominent case of personal party, the Italian Five Star Movement (5SM). Through a detailed analysis of its history, we show that the 5SM has become more institutionalized over time; however, this has especially affected its external facet, while other specific party traits have been less involved. Findings suggest that the stabilization of internal structures to connect party echelon and rank-and-file remains a challenge.