Recently, computers have been used to measure key landmarks from photocopies of upper and lower study models to increase simplicity, accuracy, and informatics. This is a comparative study to evaluate the accuracy and reliability of computer-aided space analysis. Data were collected from a series of randomly selected study models. All subjects had Angle Class I molar relationships with minor malocclusions such as crowding, rotations, or diastemas. Two investigators independently measured teeth on models with a Vernier gauge that had sharpened caliper tips. Intraexaminer and interexaminer reliability was determined at 0.2 mm. All teeth, to and including the first molars, were measured. Two photocopies of each set of models were made on a photostat machine (Xerox, Japan) and were coded. A template with a ruler was used, to allow the investigator to compensate for any reduction or enlargement error during the photocopying process. The mesiodistal sizes were measured with a digitizer, and results were processed by using a dedicated computer program. Evaluations were done in a double-blind manner. The nonparametric Wilcoxon signed rank test for paired observations to compare median differences between measurements was used. Intraexaminer digitized measurements were almost identical and differed (p < 0.0001) for only one measurement. However, interexaminer manual and digitized measurements differed significantly (p < 0.001) for 20 of the 24 teeth. Nineteen of these digitized tooth measurements were smaller. The mean arch length measurements differed by 4.7 mm (p < 0.0001) in the maxilla and by 3.1 mm (p < 0.0001) in the mandible. The difference between the manual and digitized analyses may be due to the photocopying process. The inability to accurately measure a three-dimensional study cast that has been duplicated in two dimensions, convex structure of teeth, curve of Spee, tooth inclination, and tooth position may play a role. The computer-aided measuring system is reliable, but accurate mesiodistal measurements cannot be made from photocopies of dental models. Manual measurements that use a calibrated gauge produce the most accurate, reliable, and reproducible results.