Behavioural problems in horses, in particular aggressions towards humans, are a common source of accidents in professionals (e.g. the third after dogs and bovid in veterinarians). Temperamental traits and experienceinduced aspects are commonly mentioned when evaluating horses’ reactions to humans. However, fewer studies focused on the relation between horses’ welfare and their undesirable reactions towards humans. Yet there are some recent elements suggesting that an altered welfare (from a chronic discomfort to a painful experience) could affect animals’ relational behaviour. In the present study, we hypothesized that part of the aggressiveness towards humans in horses may be linked with altered welfare. Thus, fifty nine horses from 3 riding centres were submitted to five standardized behavioural tests in order to evaluate their reactions to humans (a motionless person test, an approach-contact test, a sudden approach test with or without a saddle on the arm and halter fitting test). At the same time, a large set of potentially welfare-related indicators were recorded: health-related (e.g. vertebral problems…), postural (e.g. ears positions at rest…), physiological (e.g. stress hormones…) and behavioural (e.g. behavioural repertoire…) indicators. It appeared that about half (51%) of the horses showed at least once an aggressive reaction towards the experimenter. Interestingly, these aggressive horses displayed a characteristic ears laid back posture (Mann Whitney tests, P<0.05), were statistically more prone to suffer from vertebral problems (binomial tests, P<0.05) and had higher level of plasmatic cortisol rate (Mann Whitney tests, P<0.05). These promising results confirm a possible relation between altered welfare and part of the undesirable reactions towards humans in horses (here aggressiveness). Moreover, some of the indicators used are easy to record (e.g. ears positions at rest) and could be used to identify aggressive horses and consequently improve safety.