The aims of this longitudinal, observational study were two-fold: first, to determine in adults with sleep disorders the extent of dental and occlusal changes following the use of a mandibular advancement splint (MAS) and, second, to determine the time course of these changes. One hundred adult subjects (87 males, 13 females) diagnosed with obstructive sleep apnoea (OSA) and/or asymptomatic snoring were treated with non-adjustable MAS. At the outset each subject was randomly assigned to a group and reviewed 6, 12, 18, 24 or 30 months after placement of a splint. There were 20 subjects in each group. Craniofacial changes were measured on lateral cephalometric radiographs taken at the initial and review appointments. When the changes in all subjects were examined, the SNA, ANB angles, ANS-PNS length and face height increased, and the mandibular first molars and the maxillary first premolars significantly overerupted. Significant retroclination of the maxillary incisors and proclination of the mandibular incisors were accompanied by reductions in maxillary arch length, overbite and overjet. When the changes over time were determined, the mandibular symphysis was significantly lower at all review periods. An increase in face height and reductions in overbite and overjet were evident at 6 months, and over-eruption of the maxillary first premolars and mandibular first molars, and proclination of the lower incisors were found at 24 months. Significant positive correlations were also found between the amount of anterior opening by the appliances and changes in overbite at 24 and 30 months. The appliance used produced small, unpredictable changes in the occlusion that tended to occur after 24 months' wear. It is postulated that the changes in overbite might be lessened by keeping the bite opening to a minimum.