Background : Telomeres cap and protect the ends of chromosomes from fusion. Excessively shortened telomeres are associated with telomere dysfunction and chromosomal instability (CIN), DNA damage and an increased risk of degenerative diseases of ageing. Psychological stress has been strongly associated with accelerated telomere shortening, consistent with a wealth of evidence that chronic stress impacts negatively on health, possibly contributing to initiation of cancers, cardiovascular disease and neurodegenerative disorders such as Alzheimer's disease. Risk for these disorders is increased by deficiency in micronutrients, such as folate, an essential co-factor required for accurate replication of DNA and maintenance of methylation (epigenome) patterns, providing protection against CIN. Methods : The aim of this preliminary study was to test the hypothesis that chronic exposure to the stress hormone cortisol impacts deleteriously on telomere length (TL) and that this effect would be further aggravated by folate (Vitamin B9) deficiency. Human lymphocytes from 3 males and 3 females (aged 53±3 years) were cultured in vitro for 12 days in medium containing either 25 or 120 nM folic acid (FA), together with either 0, 550, 1300 or 3500 nM cortisol. TL (by QFISH flow cytometry), cell growth and viability were measured. Results : Cells cultured in FA-replete medium and chronically exposed to 550 or 1300 nM cortisol displayed longer TL at day 12 than cortisol-free controls (p