We assessed the repeatability and validity of the PowerRefractor and the Nidek AR-600A autorefractor. This is the first independent study conducted on adults to evaluate the performance of these instruments in a laboratory setting. Fifty subjects (23 males and 27 females) aged 16 to 61 years (mean, 37 +/- 12) participated in the study. The validity of the PowerRefractor and the Nidek autorefractor readings were determined by comparing them to subjective refraction. Measurements of refractive error were obtained from the two instruments on two separate occasions to assess their repeatability. The measured refractive error was converted into a dioptric power matrix for data analysis. No significant difference was found between the measurements obtained with the two instruments and the subjective refraction. The estimate of refractive error given by the two instruments was also found to be repeatable. In addition to measuring the refractive error, the PowerRefractor also offers the facility to measure eye position, pupil size, and dynamics of accommodation. We suggest some improvements to the PowerRefractor measurement technique to standardize its clinical use and to improve accuracy.