Consumers and health care professionals expect blood glucose monitoring systems to consistently generate results that are close to actual blood glucose levels. Numerous environmental, physiologic, and operational factors can affect system performance, yielding results that are inaccurate or unpredictable. This study examined the effect of one factor--high altitude--on the performance of seven blood glucose monitoring systems. One of the systems overestimated blood glucose results; the other six systems underestimated blood glucose values (more than the expected variance). The findings of this study support previous reports of altered blood glucose monitoring system performance at high altitude. Diabetes educators can use this information when counseling consumers who reside or who plan to visit locations at high altitude.