Administration of vaccinations via intramuscular injection is a key component of preventative medical care in horses, but development of problematic behavioral aversion to injections is quite common. It has been our clinical impression that the application of a topical anesthetic prior to vaccination may provide a simple means to reduce discomfort associated with the procedure and the resulting behavioral reactions. However, this had not been critically tested. To blindly evaluate efficacy, 78 ponies were divided into three treatment groups, each treated with either 0% control, 5%, or 10% topical lidocaine solution applied two minutes before intramuscular vaccination. Reaction scores for both the 5% and 10% lidocaine-treated groups were significantly lower than the control group. Additionally, the proportion of subjects with a reaction score greater than 1 was 2 of 25 for the 5% lidocaine solution, 5 of 26 for the 10% lidocaine solution and 15 of 27 for 0% lidocaine control. For both the 5% and 10% lidocaine solutions, the proportion differed significantly from control. The difference between the 5% and 10% lidocaine groups was not statistically significant. These findings confirm our clinical impression that application of topical anesthetic just two minutes in advance of injection can effectively reduce discomfort behavior reaction of horses.