This study examined changes in the dental arches that occur in untreated persons between late adolescence and the fifth or sixth decade of life. Longitudinal dental casts from 82 subjects were obtained as part of a recall study of subjects from the University of Michigan Elementary and Secondary School Growth Study. From the parent sample, three groups were identified. The untreated sample comprised 53 subjects (27 males and 26 females). A midadult sample of 10 persons, who had an additional set of records taken on average during their fourth decade of life also was analyzed, as was a sample of 13 subjects who received orthodontic treatment as adolescents and were about 30 years posttreatment. Measures of dental arch width, arch depth, and arch perimeter were evaluated with the aid of digital-imaging hardware and software. Incisor irregularity, curve of Spee, overjet, and overbite were measured directly from the dental casts. Statistically significant decrements occurred in arch width, depth, and perimeter. The mean decrement in any one dimension was less than 3 mm. At all times, males displayed significantly more mandibular incisor irregularity than females. In addition, the increase in mandibular incisor irregularity that occurred in male and female subjects was the same. However, irregularity did not increase in all subjects; it decreased in 3% of the males and 7% of the females. In general, overbite, overjet, and curve of Spee were stable during adulthood. Statistically significant correlations between the changes in dental arch measures could not be established.