This paper shows for the first time that Maimonides significantly relied on Avicenna's “Refutation of astrology” (Al-Išāra ilā fasād ῾ilm aḥkām al-nujūm) when composing his own refutation, the “Epistle on astrology” (῾Al Gezērat ha-kōkhabīm). Maimonides consulted a copy of Avicenna's Refutation, partially using its structure as a model for his Epistle, and adapting some of its arguments. Therefore, it may be said with certainty that Maimonides carefully read at least one of Avicenna's works, and that his Epistle is not based solely on his own previous works, the “Commentary on the Mishnah”, the “Code of Jewish law”, and the “Guide of the perplexed”.