The role of the Notch1 pathway has been well assessed in leukemia. Notch1 mutations are the most common ones in T acute lymphoblastic leukaemia patients which carry either oncogenic Notch1 forms or ineffective ubiquitin ligase implicated in Notch1 turnover. Abnormalities in the Notch1-Jagged1 system have been reported also in acute myelogenous leukaemia (AML) patients where Jagged1 is frequently over-expressed. Moreover, activating Notch1 mutations, as well, can occur in human AML and in leukemia cases with lineage infidelity. As a result, Notch1 signalling inhibition is an attractive goal in leukaemia therapy. Blockage/delay in cell differentiation and/or increase of proliferation are the main results of Notch1 signalling activation in several leukemic cell lines. Moreover, specific genes involved in cell growth control have been identified as Notch1 transcriptional targets, i.e. Cyclin D1 and c-Myc. 4-Hydroxynonenal (HNE), an aldehyde produced during lipid peroxidation, is involved in several pathological and physiological conditions, including inflammation; atherosclerosis; and neurodegenerative and chronic liver diseases. Moreover HNE has an antiproliferative/ differentiative effect in several cell lines, by affecting the expression of key genes, such as oncogenes (e.g. c-Myc, c-Myb), cyclins and telomerase. This prompted us to study the effect of HNE on Notch1 expression and its related signalling in HL-60 cells, a leukemic cell line widely used for differentiation studies. RT-PCR as well as Western blot assay showed Notch1down-regulation in HNE-treated HL-60 cells. The expression of Hes1, a Notch1 target gene, was concomitantly down-regulated by HNE treatment, reflecting Notch1 signalling inhibition. DAPT, an inhibitor of Notch activity, when added contemporary to HNE, further increased cell growth inhibition, without affecting apoptosis. Moreover, DAPT treatment reversed the HNE-induced differentiation. Overall these results suggest that Notch1 is a target for HNE and its down regulation is a key event in HNE-mediated inhibition of cell proliferation in the HL-60 cell line. By contrast our data do not support a role for Notch1 in HNE- induced differentiation or apoptosis.