The study was to determine the effect of preexisting pain on the perception of a painful stimulus. We conducted a cross-section study at an urban ED using convenience sampling. Adult patients who had a 20-g IV catheter placed as part of their ED care were eligible for the study. Patients were excluded for the following reasons: more than one IV attempt, altered mental status, visual impairment, intoxication, or a physical abnormality at the IV site. Patients were asked to indicate on a 10-cm visual analog scale (VAS) the amount of pain they had at baseline immediately before IV placement. They were then asked to indicate on a separate VAS the amount of pain caused by the IV placement. Correlation between baseline pain and pain of the IV was assessed using Pearson's rho. One hundred patients were enrolled in the study. The pain of IV placement did not differ significantly by gender, race, who placed the IV, or the location of the IV. The correlation between baseline pain and pain of the IV placement was poor (rho =.14, confidence interval:-.06-.33). The response to a standardized painful stimulus among ED patients does not correlate highly with the severity of preexisting pain.