INTRODUCTION: Pain scales such as the 100-MM Visual Analog Scale and the 10-point Numeric Rating Scale are used to describe pain intensity. The Visual Analog Scale and the Numeric Rating Scale provide accurate descriptors for a patient's perceived level of pain. But how accurate or reliable is a patient's perception of pain? METHODS: To test the relationship between the intensity of the pain stimulus and pain perception, we devised an experiment using a convenience sample of 20 healthy adult volunteers. A cutaneous nerve stimulator delivered a series of shocks of increasing intensity to the individual via a pediatric EKG electrode. The participants indicated their threshold for "intolerable pain." With use of this same level of stimulus in subsequent shocks, the participants, blinded to the amount of stimulus, were then asked to rate each shock as either "the same," "a little less," or "a little more" than the baseline stimulus. They then recorded their VAS score for each stimulus. RESULTS: "Intolerable pain" varied widely between 8 mm to 73 mm; likewise, the level of stimulus that produced this pain ranged from 4 to 9. Once a person's threshold of "intolerable pain" had been reached, 49% of the subsequent shocks were perceived as different, even though the stimulus was exactly the same. DISCUSSION: This experiment showed that (1) given the same intensity of pain stimulus, different persons have different perceptions of pain; and (2) the same intensity of pain stimulus, given to the same person repeatedly, does not result in the same self-report of pain intensity.