The changes in the dental arch dimensions that occur as a result of growth and treatment are of interest to the orthodontist and require careful consideration during treatment planning. A greater understanding of these changes could influence the patient's expectations from treatment as well as the formulation of the treatment and retention plans by the clinician. A retrospective study of the maxillary and mandibular canine and molar arch width changes in 60 patients over 20 years was carried out. Approximately half were treated orthodontically, and measurements were made on dental casts taken at four time points during the study: 1981, 1985, 1989, and 2001. Between baseline and final follow-up, the treated group demonstrated a statistically significant increase in maxillary intercanine arch width and statistically significant decreases in maxillary intermolar and mandibular intercanine and intermolar widths. No significant changes were observed for the untreated group. When comparing the orthodontically treated group with the untreated group, there was a significantly greater increase in maxillary intercanine width and a significantly greater reduction in mandibular intercanine width in the treated group over the duration of the study. No significant difference was observed between treated and untreated groups for maxillary and mandibular intermolar width changes. Sex had no statistically significant effect on these treatment differences. Type of orthodontic treatment had no effect on arch width changes within the treated group; however, the effect of tooth extraction needs further investigation.