Although vertebral problems are regularly reported on riding horses, these problems are not always identified nor noticed enough to prevent these horses to be used for work. On another hand, behavioural problems, in particular aggression towards humans, are a common source of accidents in professionals (e.g. the third after dogs and bovids in veterinarians). In the present study, we hypothesized that part of these undesirable reactions may be due to altered welfare, and possibly back pain. Thus, fifty nine horses from 3 riding centres were submitted on one hand to standardized behavioural tests in order to evaluate their reactions to humans, on another hand to chiropractic examination performed by an experienced chiropractor, who was ‘blind’ to the results of the behavioural tests and did not know the horses beforehand. The results show (1) that about half (51%) of the horses showed at least once an aggressive reaction towards the experimenter, (2) that 73% of the 59 horses were severely affected by vertebral problems, while 23% were mildly or not affected by such problems. Interestingly, horses severely affected were statistically more prone to aggressive reactions towards a human than the others, confirming a possible relation between back pain and aggressiveness. Differences could be observed between riding centres on both aspects (behavioural and vertebral) and we investigated whether differences in working conditions may explain such differences. Nineteen of these horses, corresponding to two riding centres, could be followed at work (riding lesson) using video recording: it appeared that the degree of vertebral problems identified at rest was statistically correlated with the riders’ (hands’ height, rein length and leg position) and horses’s (neck height and curve) attitudes at work. Riders and horses attitudes clearly differed between centres. These promising results are part of a larger scale study on horses’ welfare and suggest fruitful new lines of research based on crossing of chiropractics and behavioural studies.