Objectives: This study aimed to assess the predictors of allergic sensitisation to titanium and nickel in patients undergoing orthodontic treatment. Methods: A total of 250 patients undergoing orthodontic treatment were invited to participate, and 235 were analysed (67% females). A patch test was performed using nickel sulphate, titanium, titanium dioxide, titanium oxalate, titanium nitride, and petrolatum as control. In addition, clinical signs of the oral mucosa, gingiva, tongue, lips, and allergological history were assessed. Results: The predictors of metal allergic sensitisation in patients undergoing orthodontic treatment were adult age (OR = 2.6 ; 95% CI: 1.2- 5.5 ; p = 0.016), female sex (OR = 3.0 ; 95% CI: 1.1-7.9 ; p = 0.025), exfoliative cheilitis (OR = 4.8 ; 95% CI: 1.9-12.4 ; p = 0.001), history of contact hypersensitivity (OR = 7.0 ; 95% CI: 1.3- 35.4 ; p = 0.025), history of contact hypersensitivity to metals (OR = 8.3 ; 95% CI: 1.4- 50.2 ; p = 0.021), and piercings (OR = 5.4 ; 95% CI: 2.1-13.9 ; p = 0.001). When predictors were analysed separately for these two metals, titanium sensitisation predictors were contact hypersensitivity to metals and piercing, whereas nickel sensitisation predictors were age and piercing. Conclusion: A positive patch test alone cannot draw definite conclusions regarding allergy. However, metal allergies in patients with orthodontic appliances could be considered in cases of previous contact hypersensitivity, previous reactions to metals, exfoliative cheilitis, and piercing.