In his 1978 'Islamic Medicine', Manfred Ullmann underscored the serious terminological confusion about the Greek term φρενίτις in the Arabic sources*1*. According to Ullmann, the disease known in Hippocratic medicine as phrenitis was translated by Arabic authors into either birsām or sirsām, two Persian loan-words. Ulmann stated that some authors understood the two terms to designate different illnesses; al-Rāzī, however, used them interchangeably. Ullmann attributed the confusion of the two words to their frequent use in pre-Islamic poetry. Michael Dols*2* echoed Ullmann's remarks stating that Arabic translators did not create in this case a clear, shared nomenclature. More recently, Peter Pormann pointed out that the two terms were used ‘nearly synonymously’*3* in Arabic medicine to designate an acute illness with fever caused by an inflammation of the meninges. The present paper aims at rectifying our understanding of the usage of the terms birsām and sirsām in selected Arabic sources. The period under scrutiny ranges from Ḥunayn's translations of Galen (9th c. AD) up to Ibn al-Quff's commentary on the Hippocratic Aphorisms (13th c. AD). With the aid previously unpublished sources, I show how influential Arabic writers consistently used sirsām and birsām to designate two pathologies which displayed the same symptoms, but had different aetiologies. In particular, commentators since Ḥunayn used the two terms to discern between the affected parts when delirium and fever arose. I demonstrate that, contrary to what is commonly thought, Ḥunayn's understanding of phrenitis did not overlap with Galen's and I formulate a hypothesis on the reason for this discrepancy. *1*Ullmann, M. Islamic Medicine. Edinburgh University Press (1978). 29 *2*Dols, M. Majnūn: the Madman in Medieval Islamic Society. Oxford (1992). 57 *3*Pormann, P. "Theory and Practice in the Early Hospitals in Baghdād: al-Kaskarī on Rabies and Melancholy." Zeitschrift für Geschichte der Arabisch-Islamischen Wissenschaften 15(2003): 197-248. 217