This article presents the Mafia Index (MI), an index measuring the presence of mafias at the provincial level. In the abundant literature on Italian mafias, relatively few studies have attempted to measure the presence of mafias across the country. A review of previous attempts points out the limitations and methodological shortcomings of existing measurements. The study provides an operational definition of 'mafia' and selects the most appropriate indicators and variables according to multiple criteria. The MI combines data on mafia-type associations, mafia murders, city councils dissolved for infiltration by organised crime, and assets confiscated from organised crime and covers the period between 1983 and 2009. The MI highlights not only the strong concentration of the mafias in their original territories but also their significant presence in the central and northern provinces. This confirms that mafias should not be regarded as typically Southern Italian phenomena, but rather as a national problem. (Published Abstract)