Aim: To compare the 0.018-inch and 0.022-inch conventional pre-adjusted orthodontic bracket slot systems in terms of the effectiveness of levelling and alignment stage of orthodontic treatment. Design: Prospective, multi-centre randomised clinical trial. Setting: This was undertaken in the secondary care hospital environment in Tayside NHS in the United Kingdom. Subjects and methods: One hundred and five orthodontic patients were randomly allocated to treatment with either the 0.018-inch bracket slot (n= 52) and 0.022-inch bracket slot (n=53) Victory conventional pre-adjusted bracket systems (3M Unitek). The patients were treated in three centres in secondary care hospitals Tayside NHS, United Kingdom. The levelling and alignment stage of treatment was assessed from the start of treatment until the ligation of the working archwire for each bracket slot system (0.016x0.022 stainless steel for the 0.018-inch group and 0.019x0.025 stainless steel for the 0.022-inch group). Periapical radiographs were taken before the start of treatment and after 9 months in treatment for the maxillary central incisors to assess orthodontically-induced inflammatory root resorption (OIIRR). The “Smiles better” questionnaire was completed by the participants at 6 months from the start of treatment. Primary outcome measures: The duration of the levelling and alignment stage of orthodontic treatment in the maxillary and mandibular arches. Secondary outcome measures: The number of scheduled appointments for the levelling and alignment stage of orthodontic treatment in the maxillary and mandibular arches, OIIRR at 9 months from the start of treatment using periapical radiographs and patient perception of wearing orthodontic appliances. Results: The data from 92 patients (mean age 19.55 years) were analysed after the completion of their levelling and alignment stage of orthodontic treatment. An ANOVA test showed no statistically significant difference in the duration or number of scheduled appointments for the levelling and alignment stage in the maxillary and mandibular arches between the two appliance groups. Non-parametric statistical test showed no statistically significant difference in the severity of OIIRR and patient perception of wearing orthodontic appliances between the two study groups except for the soreness of teeth, where more patients in the 0.022-inch group experienced significant teeth soreness than the 0.018-inch group. Multiple regression analysis determined that 49.6% of the variance in the duration of levelling and alignment duration for the maxillary arch can be explained by five factors: alignment of ectopic tooth, scheduled appointment intervals, gender, bracket slot size system and the number of failed scheduled visits. For the mandibular arch, 50.8% of the variance in the levelling and duration of alignment can be explained by three factors: scheduled appointment intervals, arch irregularity and the number of debonded brackets. Conclusions: There is no difference in the effectiveness of the levelling and alignment stage of orthodontic treatment between the 0.018-inch or 0.022-inch conventional bracket slot systems except for the soreness of teeth.