The reliability of measurements of passive ankle dorsiflexion obtained in children using both the Universal Goniometer (UG) and an adapted Biplane Goniometer (BPG) was studied. Twenty-five children were measured from each of 3 diagnostic categories: juvenile rheumatoid arthritis (JRA), cerebral palsy (CP), and nondisabled (ND) (N = 75). Five pairs of raters each measured five children from each diagnostic category. Generalizability theory provided a framework to determine the standard error of measurement (SEM) associated with each goniometer under different situations. Results revealed that the amount of error associated with each goniometer was similar when both rater and time were random, and ranged from +6.50 for the CP group to +4.40 for the JRA group. Error was reduced by almost half in all diagnostic groups by fixing either occasion or rater, or by taking the average of one rater's two consecutive measurements. Clinically, the goniometers can be used interchangeably and averaging of measurements is recommended.