IntroductionIt is a matter of clinical experience that psychological factors, in particular stress, can exert notable effects on primary headaches. Several studies have shown that stress is one of the most common trigger factors for headache, both in migraine and in tension-type headache [1][2][3][4][5]. It is reasonable, therefore, that stress may exert effects on the clinical evolution of these headaches. For example, stressful events may increase headache frequency or promote transformation of an episodic headache into a chronic form. A chronic headache is one present on average more than 15 days per month [6]. A transformed headache is an originally episodic headache that developed into a chronic one.A stressful event is an environmental situation or psychological trauma that compromises or threatens wellbeing. Several models have been proposed to explain how stress can influence headache. More recent theories do not consider stress as a purely exogenous factor but recognise that stressful events can induce objective biological and psychological changes.Stress acts on the body via the endocrine system, autonomic nervous system and immune system [7][8][9]. These systems act as biological integrators in the body, functioning to maintain homeostasis. In turn they are influenced by such factors as genetic makeup or constitution, psychobiological imprinting and the external environment (the sum of physical, emotional and social stimuli). Stressful events affect the brain through inputs from the cortex, subcortical regions, and sensory organs, and from the endocrine system, provoking reactions mediated, principally, by the hypothalamo-hypophyseal-adrenal axis (CRH) and by the locus coeruleus (norepinephrine). The effects of these multifarious hormonal and neurotransmitter changes vary with the duration of the stressful stimulus. Prolonged and repeated exposure to stressful stimuli may lead to permanent functional changes and even to anatomical damage, and can thus assume the role of a pathogenetic agent or illness precursor. Abstract The aims of this study were to assess how stress affects chronic headaches, and in particular to determine whether events play a role in the transformation of an episodic headache into a chronic form. A population of 267 Italian patients with chronic headache (headache present on average more than 15 days per month) was studied. Our results confirm a triggering role of stress on headache. We also found that episodic headache (migraine in most patients, 74.1%) preceded the development of a chronic form in about 90% cases. Among these patients, in 44.8% a stressful event correlated with the transformation. Analysis of these events revealed that minor events played a greater role than major life events, suggesting that patients with transformed headache are characterised by a different way of reacting to stress.