The purpose of this study is two-fold. First, it aims to show whether there are differences in lexical density between two historical varieties of Arabic, namely, Late Middle Arabic and Modern Standard Arabic. Second, it seeks to find out the method of measuring lexical density that best suits the Arabic language, with its orthographic and morpho-syntactic peculiarities. To this end, it compares the two main methods for measuring lexical density, Ure’s and Halliday’s, and discusses the difficulties that arise when each of them is applied to Arabic, suggesting solutions where possible. Each method is then used to measure lexical density in a selection of texts representing the two varieties of Arabic and belonging to the historical narrative genre. The results of the comparison indicate that the two varieties display different degrees of lexical density, though the two methods of measurement yield opposing results. However, it is shown that Halliday’s method is more appropriate for Arabic and that it consistently reveals significant differences between the historical varieties as represented by the texts analysed.