This paper first examines surviving testimonies on the doctrine of the Great Year in Heraclitus and attempts to demonstrate the reliability of Aëtius’ version handed down by the mss., according to which the Great Year is equal to 18,000 solar years. On the basis of such evidence it is also possible to newly examine Diogenes of Babylon’s views about this topic. In the second part, the paper better defines the relationship between the Great Year and the theory of cosmic conflagration. It argues that in the sources at our disposal there are enough elements to credit Heraclitus with the doctrine of Ekpyrosis , although, in all probability, the philosopher never provided an in-depth description of it. Finally, the same problem is analysed in relation to Diogenes of Babylon. In the most mature phase of his career, this Stoic departed from the Ekpyrosis doctrine of the early Stoics and especially of his master Chrysippus. The paper formulates some hypotheses on the reasons for this choice (arguing that it most probably reflects an original attitude towards Heraclitus on Diogenes’ part) and highlights that it was hardly an isolated, ‘heterodox’ stance during the final phase of the early Stoa.