Notch signalling plays an important role in hematopoiesis and in the pathogenesis of T‐ALL. Notch is known to interact with Ras and PTEN/PI3K (phosphoinositide‐3 kinase)/Akt pathways. We investigated the interaction of Notch with these pathways and the possible reciprocal regulation of these signalling systems in T‐ALL cells in vitro. Our analyses indicate that the PI3K/Akt pathway is constitutively active in the four T‐ALL cell lines tested. Akt phosphorylation was not altered by the sequestration of growth factors, that is, Akt activation seems to be less dependent on but not completely independent of growth factors, possibly being not subject to negative feedback regulation. PTEN expression was not detected in 3/4 cell lines tested, suggesting the loss of PTEN‐mediated Akt activation. Inhibition of the PI3K/Akt pathway arrests growth and enhances apoptosis, but with no modulation of expression of Bax‐α and Bcl‐2 proteins. We analysed the relationship between Notch‐1 and the PI3K/Akt signalling and show that inhibition of the Akt pathway changes Notch expression; Notch‐1 protein decreased in all the cell lines upon treatment with the inhibitor. Our studies strongly suggest that Notch signalling interacts with PI3K/Akt signalling and further that this occurs in the absence of PTEN expression. The consequences of this to the signalling outcome are yet unclear, but we have uncovered a significant inverse relationship between Notch and PI3K/Akt pathway, which leads us to postulate the operation of a reciprocal regulatory loop between Notch and Ras‐PI3K/Akt in the pathogenesis of T‐ALL. J. Cell. Biochem. 103: 1405–1412, 2008. © 2007 Wiley‐Liss, Inc.