We conducted a study to determine the change in nerve conduction testing after steroid injection in patients with carpal tunnel syndrome (CTS). One hundred forty-five patients with suspected CTS were targeted for this study. Twenty-seven patients underwent testing before and after injection. Repeat studies were performed 4 to 6 weeks after injection. All data from the electrodiagnostic studies were entered into a database and used for comparison. Before injection, mean (SD) distal motor latency (DML) was 5.01 (0.9) ms, and mean (SD) peak sensory latency (PSL) was 5.01 (0.88) ms. After injection, mean (SD) DML was 4.82 (0.7) ms, and mean (SD) PSL was 4.69 (0.66) ms. Mean (SD) difference between preinjection and postinjection DML was 0.187 (0.45) ms, and mean difference between preinjection and postinjection PSL was 0.319 (0.48) ms. Both differences were statistically significant (paired t test). Our study results showed a statistical difference between testing done before and after steroid injection. These results indicate that injections given before electrodiagnostic testing alter results and may affect patient management.