The Historia Augusta is a notoriously problematic text. It consists of a series of thirty biographies of emperors, Caesars and usurpers that cover the years from 117 to 285 AD. It is presented in the manuscript tradition as the collected work of six authors, writing at the beginning of the fourth century AD. However, we now know that the Historia Augusta was written by one author at the end of the fourth century. Nonetheless it is still no less difficult to interpret because its pages are beset by a bizarre mixture of fact, fiction, fabricated sources, jokes and literary allusion. Alongside this interpretative challenge is another problem: a large lacuna in the text which has resulted in the loss of the biographies covering the period from 244 to 260 AD. Anthony Birley has argued extensively that this lacuna, rather than a true loss of text, represents an additional forged textual element. Birley suggests that this period was deliberately omitted by the Historia Augusta’s author for two main reasons: to avoid covering the contentious emperors Decius and Valerian, and to give the work the appearance of greater antiquity. Birley’s theory has received widespread acceptance in recent scholarship on the Historia Augusta. This paper is a critical analysis of his theory that provides evidence that, to the contrary, the nature of the lacuna is far from clear.