We determined 1140 genes as the ‘perfect’ core genome in Legionella species. Previous estimates of core genome sizes are available only for Lp1 where 815 and 2745 genome comparisons estimated the size to be 2405 and 2173 genes, respectively. A very recent study18 calculated the presence of 3120 core genes based on the comparison of 69 strains of Lp1, which were part of outbreaks occurring in Alcoy, Spain, over 11 years, in addition to nine Lp1 reference strains. This latter analysis did not capture the ‘true’ estimate of core genes because a distribution of Legionella species was not included.